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>2 


DEPARTMENT  OF  THE  INTERIOR 

UNITED  STATES  GEOLOGICAL  SURVEY 

GEORGE  OTIS  SMITH,  Director 


FOLIO  193-FlELD  EDITION 


"Q^^r  V5"^r 


GEOLOGIC  ATLAS  OF  THE 
UNITED   STATES 


San   Francisco   Folio 


BY 


ANDREW  C.  LAWSON 


WASHINGTON 

U.  S.  GEOLOGICAL  SURVEY 

1915 


stack 
Annex 

6 
50:3^ 


CONTENTS. 

Page. 

Geography 9 

Situation  and  general  divisions  of  the  area 9 

Area  east  of  San  Francisco  Bay 10 

Relief 10 

Drainage 11 

Soil 13 

Vegetation 14 

Climate 15 

San  Francisco  Peninsula 16 

Relief  and  drainage 16 

Shore  lines 18 

Soil 18 

Vegetation 19 

Climate 20 

Marin  Peninsula 20 

Relief 30 

San  Andreas  rift  valley 22 

Point  Reyes  Peninsula 22 

Soil 23 

Vegetation 23 

Climate 24 

Bay  of  San  Francisco 25 

Geology 28 

Stratigraphy  and  areal  geology 38 

Geologic  formations  of  the  middle  Coast  Ranges 28 

Pre-Franciscan  rocks 31 

Character  and  distribution .. 31 

Gavilan  limestone 31 

Quartz  diorite  ("Montara  granite") 31 

Jurassic  (?)  rocks 34 

Franciscan  group  and  associated  igneous  rocks 34 

Character 34 

Stratigraphy 34 

Cahil,  Marin,  and  Bonita  sandstones 35 

Character 35 

Calera  limestone  member  of  Cahil  sandstone 38 

Sausalito  and  Ingleside  cherts . 40 

General  character 40 

Petrographic  features 41 

Fossils 43 

ij                                    Distribution  and  tiiickness 44 

(3) 


(ieology — Continued.  Page. 
Stratigraphy  and  areal  geology — Continued. 
JuraBsic  (?)  rocks — Continued. 

Franciscan  group  and  associated  igneous  rocks — Continued. 

Conditions  of  sedimentation 44 

Contemporaneous  volcanic  rocks 46 

Intrusive  rocks  associated  with  the  Franciscan  group.  47 

Peridotite  and  serpentine 47 

tSilica-carbonate  rock 50 

Spheroidal  basalt  and  diabase 50 

Metaniorphic  rocks 52 

Age  of  the  Franciscan  group 54 

Cretaceous  system 57 

Distribution 57 

Lower  Cretaceous  (Shasta)  series 58 

Knoxville  formation 58 

Upper  Cretaceous  series 60 

Chico  foriuation 60 

Oakland  conglomerate  member 60 

Upper  part  of  Chico  formation 62 

Fossils 63 

Tertiary  system 64 

Eocene  series 64 

Subdivisions 64 

Martinez  formation 65 

Character  and  distribution 65 

Fossils 66 

Rocks  of  San  Pedro  Point 67 

Tejon  formation 68 

Distribution  and  character 68 

Fossils 69 

Miocene  series 70 

Monterey  group 70 

Petrographic  character 70 

Relations  to  older  formations 73 

Subdivisions 75 

Sobrante  sandstone 78 

Fossils  of  lower  faunal  zone 78 

Claiemont  shale 79 

Oursan  sandstone 80 

Tice  shale . ^ 80 

Hambre  sandstone ,._.  81 

Rodeo  shale 81 

Fossils  of  juiddle  faunal  zone 81 

liriones  sandstone 82 

( 'haracter  and  distribution 82 

Hercules  shale  member 82 

Fossils  of  upper  faunal  zone ^^3 

Partly  differentiated  Monterey  strata  in  the  Con- 
cord quadrangle 83 

Undifferentiated  Monterey  sti-ata  in  the  Tamalpais  -^ 

q  uad  ran  gle 84 

Hasalt          .   __. 85 


Geology — Continued.  Page. 
Stratigraphy  and  areal  geology — Continued. 
Tertiary  system — Continued. 
Miocene  series — Continued. 

San  Pablo  formation 86 

(jreneral  features 86 

Distribution 86 

Thickness 87 

Age 87 

Fossils 88 

Pliocene  series 89 

Leona  rhyolite  __. 89 

Distribution  and  chax'acter 89 

Chemical  composition .. 91 

Field  relations 92 

Age 92 

Northbrae  rhyolite 93 

Distribution __. 93 

Correlation 93 

Petrographic  features 93 

Pinole  tuff 94 

General  features 94 

Peti'ogi-aphic  character 95 

Distribution 96 

Orinda  formation 97 

General  features 97 

S*tratigraphic  relations 98 

Fossils 99 

Age 100 

Merced  formation 101 

General  character  and  distribution .__  101 

Stratigraphic  relations 101 

Fossils L 103 

Berkeley  group ^ 104 

General  description 104 

Moraga  formation ^ 105 

Siesta  formation 106 

Bald  Peak  basalt . 106 

Relations  of  the  group  to  adjacent  formations 106 

Tertiary  and  Quaternary  deposits 107 

Santa  Clara  formation 107 

Quaternary  system 108 

Pleistocene  series 108 

Campus  formation 108 

Alameda  formation , 109 

San  Antonio  formation .  Ill 

Merritt  sand 112 

Terrace  gravel . 113 

Recent  series ; 113 

Temeseal  formation . 113 

Other  recent  deposits 114 

Terrace  deposits 114 

Travertine 114 


6 

Geology — Continued.  Page. 
Stratigraphy  and  areal  geology— Continued. 
Quaternary  system — Continued. 
Recent  series — Continued. 

Other  recent  deposits — Continued. 

Dunes 114 

Salt-marsh  deposits 114 

Structure 115 

General  features 115 

Montara  block 116 

Faults 116 

Folds : 118 

San  Francisco-Marin  block -._  131 

Faults 121 

Folds 123 

Berkeley  Hills  block 125 

Faults 125 

Folds 133 

(ieologic  history 138 

Pre-Franciscan  time 138 

Franciscan  epoch 139 

Knoxville  epoch 140 

Chico  epoch 141 

Martinez  epoch 142 

Tejon  epoch 143 

Monterey  epoch  _. 143 

San  Pablo  epoch A 145 

Merced  epoch 146 

Berkeley  epoch 147 

Deformation  at  close  of  Tertiary  period 147 

Campus  epoch ■. 148 

Alameda  epoch 148 

Post- Alameda  diastrophism 148 

San  Antonio  epoch 149 

Merritt  and  Temescal  epoch 149 

Recent  uplift  and  depression 150 

Changes  in  drainage 152 

Economic  geology 154 

Available  resources 154 

Water . 154 

Brick  clays _ 159 

Orinda  formation 159 

Siesta  formation . 160 

Alluvial  and  marsh  clay 160 

Shale  for  making  bricks  and  cement IGI 

Limestone 161 

Crushed  rock 162 

Gravel  and  sand. . 163 

Abrasive 164 

Greensand _ 164 

Oil 164 

Salt...                                                                                                        __.  165 

Pyritc             165 


V 

Economic  geology — Continued.  Page. 

Quicksilver 166 

Mangane'e 16T 

Lead 169 

Gold 169 

Copper I'i^" 

Asbestos,  chroniite,  talc,  and  niagnesite ITO 

Minerals IW 

Earthquakes  and  construction 171 

Literature l^-") 


ILLUSTRATIONS. 


Maps,  etc.  (in  pocket): 
Topographic  maps. 

Colnnmiar  section  (on  back  of  Tamalpais  topographic  map). 
Areal-geology  maps. 
Structure-section  sheets. 
Plates  (at  end  of  text) : 

I.  Steeply  tilted  rocks  of  Ban  Pedro  Point,  San  Mateo  quadrangle. 
II.  Western  front  of  the  Berkeley  Hills,  viewed  from  the  north  across 
the  canyon  of  Strawberry  Creek. 

III.  Calera  limestone  member  of  Cahil  sandstone,  Franciscan  group, 

Calera  Point,  San  Mateo  quadrangle. 

IV.  San  Francisco  Bay,  from  the  Berkeley  Hills. 

V.  Ellipsoidal    structure    in   intrusive    basalt,    Hunter  Point,    San 

Francisco. 
VI.  Ellipsoidal  basalt  intrusive  into  thin-bedded  radioiarian  chert  of 
Franciscan  group,  Hunter  Point,  San  Francisco. 
VII.  Thin-bedded    chert    and    shale    of    the    Claremont    formation, 
Monterey  group,  Clai-emont  Canyon,  Berkeley  Hills. 
VIII.  Minutely   folded   thin-bedded   radiolarian  chert  of    Franciscan 
group  exposed  in  quarry  in  Golden  Gate  Park,  San  Francisco. 
IX.  San   Andreas   rift   valley;   view  southeastward   toward  Crystal 

Springs  Lake,  San  Mateo  quadrangle. 
X.  Trace  of   San   Andreas  fault,    made   by   the   movement   which 
caused  the  earthquake  of  1906. 
Figures:  •  Page. 

1.  Index  map  of  central  California 10 

2.  Map  of  vicinity  of   San  Francisco  Bay,  showing  topographic 

features  and  the  geomorphic  divisions  resulting  from  the 
adjustment  by  tilting  of  great  fault  blocks 10 

3.  Outline  map  of  the  Tamalpais,  San  Francisco,  Concord,  San 

Mateo,   and    Haywards  quadrangles,   showing  the  limits  of 

the  great  fault  l)locks,  the  larger  faults,  and  the  axes  of  folds      11") 

4.  Outline  map  of  the  western  slope  of  the  Berkeley  Hills  in  the 

southwestern  jjart  of  the  Concord  quadrangle  and  adjacent 
parts  of  the  San  Francisco  and  Haywards  quadrangles, 
showing  the  deflection  of  streams  by  the  longitudinal  rift 
valley  of  the  Haywards  fault  zone 127 


DESCRIPTION   OF   THE   SAN  FRANCISCO 

DISTRICT. 


By  Andrew  C.  Lawson." 


GEOGRAPHY. 

SITUATION    AND    GENERAL    DIVISIONS    OF   THE    AREA. 

The  five  sheets  of  the  San  Francisco  folio — the  Tama  1  pais, 
Ban  Francisco,  Concord,  San  Mateo,  and  Haywards  sheets — 
map  a  territory  lying  between  latitude  37°  30'  and  38°  and 
longitude  122°  and  122°  45'.  Large  parts  of  four  of  these 
sheets  cover  the  waters  of  the  Bay  of  San  Francisco  or  of  the 
adjacent  Pacific  Ocean.  (See  fig.  1.)  Within  the  area  mapped 
are  the  cities  of  San  Francisco,  Oakland,  Berkeley,  Alameda, 
San  Rafael,  and  San  Mateo,  and  many  smaller  tOAvns  and  vil- 
lages. These  cities,  which  have  a  population  aggregating  about 
750,000,  together  form  the  largest  and  most  important  center 
of  commercial  and  industrial  activity  on  the  west  coast  of  the 
United  States.  The  natural  advantages  afforded  by  a  great 
harbor,  where  the  railways  from  the  east  meet  the  ships  from  all 
ports  of  the  world,  have  determined  the  site  of  a  flourishing- 
cosmopolitan,  connnercial  city  on  the  shores  of  San  Francisco 
Bay.  The  bay  is  encircled  by  hilly  and  mountainous  country 
diversified  by  fertile  valley  lands  and  divides  the  territory 
mapped  into  two  rather  contrasted  parts,  the  western  part  being 
again  divided  by  the  Golden  Gate.  It  will  be  convenient  to 
sketch  the  geographic  features  under  four  headings — (1)  the  area 
east  of  San  Francisco  Bay;  (2)  the  San  Francisco  Peninsula; 
(3)  the  Marin  Peninsula;  (4)  San  Francisco  Bay.  Figure  2 
shows  the  topography  of  this  general  region. 

"  Seo  note  on  page  i^. 

(9) 


10 


Montcj-e  y{^ 


121° 


120 


Figure  1. — Index  map  of  central  California. 

The  location  of  the  five  quadrauKles  described  in  the  San  Francisco  folio  is  shown  by  the  darker 
ruling  (No.  193).  Published  folios  describing  other  quadrangles  are  indicated  by  lighter 
ruling  and  the  proper  serial  numbers  and  are  included  in  the  numerical  list  on  tlie  back  cover 
of  this  folio. 

AREA    EAST    OF   SAN    FRANCISCO    BAY. 


Relief. — The  area  east  of  San  Francisco  Bay  embraces  a  belt 
of  billy  country  lying  between  the  bay  and  the  western  flank 
of  Mount  Diablo.  The  ridges  trend  generally  northwest  and 
southeast.  Portions  of  two  wide  valleys  lie  within  this  area. 
One  of  these  is  the  valley  of  San  Francisco  Bay,  on  whose 
shores  stand  the  cities  of  Berkeley,  Oakland,  and  Alameda;  the 
other  is  Ygnacio  Valley,  which  occupies  the  northeast  corner 
of  the  Concord  quadrangle  and  extends  with  a  very  flat  slope 
northward  beyond  the  limits  of  the  quadrangle  to  the  shores  of 
Suisun  Bay.  The  southern  extension  of  this  valley,  up  the 
dniinago  line  of  Wnlnut  Creek,  is  the  well-defined  flat-bottomed 


V 


Figure  2.  —Ma 


The  eastern  boundi 
of  the  Berkeley 
The  submerged 
land  was  siibme 


e 
>f 
)f 
e 
a 
e 
)f 


Figure  2.  —Map  of  vicinity  of  San  Francisco  Bay,  showing  topographic  features  and  the  geomorphic  divisions  resulting 
from  the  adjustment  by  tilting  of  great  fault  blocks. 

The  eastern  boundary  of  the  Berkeley  Hills  block  is  not  sharply  deflced;  it  may  be  drawn  along  a  general  zone  of  overthrust  folding  and  faulting  just  e«st 
of  the  Berkeley  HUls.  on  which  the  Mount  Diablo  thrust  block  has  ridden  westward.  This  thrust  movement  antedated  the  tilting  of  blocks  to  the  west 
The  submerged  bar  outside  the  Golden  Gat«  originated  as  a  delta  deposit  of  the  stream  that  flowed  in  the  old  valley  of  San  Francisco  Bay  before  tJiir 


The  five  quadrangles  described  in  this  folio  are  shown  by  heavier  lines  than  the  b 


s  of  other  i|uadranglei 


C 


11 


San  Kanion  Valley,  which  sharply  separates  tlie  belt  of  hills 
above  mentioned  from  Mount  Dial)lo,  the  greater  mass  of 
which  lies  farther  east,  in  the  adjacent  quadrangle.  The  domi- 
nant range  of  hills  is  that  which  forms  the  southwestern  limit 
of  the  belt  and  which  immediately  overlooks  the  Bay  of  San 
Francisco.  The  culminating  point  on  this  range  is  Bald  Peak, 
east  of  Berkeley,  which  stands  at  an  altitude  of  1930  feet  above 
sea  level.  Otiier  peaks  whose  altitudes  afford  an  idea  of  the 
general  height  of  the  range  are  Grizzly  Peak,  1759  feet; 
Round  Top,  1750  feet;  and  Redwood  Peak,  1608  feet.  This 
range  is  commonly  referred  to  as  the  Berkeley  Hills,  although 
the  area  to  which  that  term  is  applicable  appears  to  be  rather 
vaguely  defined.  It  is  also  often  referred  to  as  the  Contra 
Costa  Hills,  but  this  term  apparently  applies  more  properly 
to  the  broad  group  of  hills  between  the  Bay  of  San  Francisco 
and  Mount  Diablo.  A  rather  well  defined  line  of  valleys, 
including  San  Pablo  and  Moraga  valleys,  separates  this 
dominant  range  from  the  more  eastern  portion  of  the  hilly 
belt.  The  hills  thus  lying  between  the  dominant  ridge  and 
Ygnacio  and  Ramon  valleys  show  a  less  pronounced  linear 
trend,  are  much  more  mature  in  their  geographic  expression, 
and  in  general  are  much  lower.  The  culminating  points  in 
this  part  of  the  belt  are  Rocky  Ridge  (2000  feet)  on  the  south, 
and  the  Briones  Hills  (1432  feet)  on  the  north. 

In  the  Haywards  quadrangle  there  is  a  notable  sag  in  the 
longitudinal  profile  of  the  range,  and  at  Haywards  there  is  a 
gap  at  the  level  of  the  alluvial  plain,  which  here  stands  about 
100  feet  above  sea  level.  This  gap  is  the  outlet  of  a  remark- 
ably open  low  valley  known  as  Castro  Valley. 

Drainage. — The  drainage  of  the  area  includes  several  features 
worthy  of  special  mention  : 

1.  The  dominant  ridge  of  the  hill  belt  does  not  form  the 
divide  that  separates  the  waters  flowing  directly  to  the  Bay  of 
San  Francisco  from  those  flowing  to  Suisun  Bay  by  way  of 
Walnut  Creek,  in  San  Ramon  and  Ygnacio  valleys.  The 
divide  runs  through  tlie  center  of  the  belt  of  hills,  so  that  a 
very  considerable  portion  of  the  lower  ground  northeast  of  the 
dominant  ridge  is  drained  either  through  or  around  the  end  of 


12 


the  ridge.  The  soiitliern  half  of  the  range  drains  through 
gaps  in  the  ridge,  which  form  outlets  to  the  bay  for  San  Lean- 
dro  and  San  I^orenzo  creeks.  Tlie  northern  half  is  drained 
by  San  Pablo  Creek,  which  flows  around  the  end  of  the  main 
ridge,  and  by  Pinole  Creek,  which  flows  to  San  Pablo  Bay. 

2.  A  second  feature  of  the  drainage  of  especial  interest  is 
that,  though  the  streams  on  the  northeast  side  of  the  dominant 
ridge  are  manifestly  subsequent,  those  draining  the  southwest- 
ern slope  of  the  same  ridge  directly  to  the  bay  are  as  mani- 
festly consequent,  presenting  a  condition  which  suggests  that 
the  slope  to  San  Francisco  Bay  is  of  more  recent  origin 
than  the  maturely  dissected  hill  country  farther  northeast — a 
suggestion  which  is  more  fully  discussed  in  this  text,  under 
the  heading  "Structure." 

3.  A  third  feature  is  the  prevailing  alluviation  of  the  valley 
bottoms  and  the  steep-sided  stream  trenches  cut  in  the  alluvium, 
which  do  not  as  a  rule  reach  the  bedrock,  clearly  indicating 
that  the  former  conditions  in  this  area  favored  more  vigorous 
downward  corrasion  by  the  streams,  at  a  time  when  the  can- 
yons and  valleys  were  deeper,  and  that  with  the  passing  of 
these  conditions  others  ensued  which  reduced  the  transporting 
power  of  the  streams  and  caused  them  to  drop  their  load  of 
detritus  in  the  bottoms  of  the  canyons  and  valleys,  making 
these  flat  and  broad,  as  well  as  helping  to  give  the  country  its 
geomorphically  mature  aspect  and  adding  to  its  agricultural 
value.  Since  these  flat-bottomed  valleys  were  thus  formed 
there  seems  to  have  been  a  slight  but  distinct  tendency  toward 
a  recurrence  of  tlie  older  conditions,  indicated  by  the  trench- 
ing of  the  valley  floors,  but  this  trenching  may  be  due,  in  part 
at  least,  to  the  disturbance  of  natural  conditions  caused  by 
culture. 

Attention  may  be  called  also  to  the  rather  noteworthy  con- 
vergence of  drainage  in  Castro  Valley  near  Haywards.  Most 
of  this  drainage  is  carried  l)v  San  Lorenzo  Creek  throuR'h  a 
pronounced  ))i'eak  in  the  Berkeley  Hills. 

The  only  other  notable  feature  of  the  drainage  is  the  fact 
tliat  the  stream  flowing  in  San  Bamon  Valley,  the  largest, 
most  mature,  and  broadest  valley  in  the  Concord  quadrangle, 


13 


has  cut  tliroii.i;li  tlie  alluvium  on  the  vallev  floor  at  several 
places,  particuhn-ly  from  Alamo  to  the  village  of  Walnut 
Creek,  where  it  runs  on  bedrock. 

Soil. — The  soils  of  the  country  may  be  divided  into  two 
classes.  The  soil  on  the  hillsides  and  ridge  tops  above  the 
level  of  the  valley  floors  is  sedentary — that  is  to  say,  it  has 
been  formed  in  place  by  the  chemical  and  mechanical  disinte- 
gration of  the  underlying  rocks.  This  soil  has  l)een  modified 
chiefly  by  tlie  abstraction  of  certain  constituents  which  nour- 
ished the  generations  of  plants  that  have  grown  upon  the  sur- 
face and  by  the  addition  of  organic  matter  formed  by  the  decay 
of  the  same  vegetation.  On  these  hill  slopes  earthworms  ai-e 
uncommon,  probably  because  the  soil  is  very  dry  and  parched 
during  the  summer,  so  they  have  not  aided  in  turning  over  and 
mixing  the  soil  and  in  thus  making  it  more  useful  for  agricul- 
ture. This  work,  however,  lias  been  performed,  probably 
with  equal  efficiency,  by  several  kinds  of  burrowing  mammals, 
such  as  the  gopher  (Thomomys)  and  the  ground  squirrel  (Sper- 
mophilus).  These  animals  formerly  infested  the  region  in 
great  numbers  and  have  persisted  there  until  very  recently, 
in  spite  of  the  eff'orts  of  the  farmers  to  destroy  them,  but 
during  the  last  few  years,  by  more  systematic  efforts,  the 
health  authorities  have  almost  completely  exterminated  them, 
because  they  are  regarded  as  a  menace  to  the  public  health  as 
propagators  of  the  bubonic  plague  through  the  fleas  wdiich  infest 
them. 

These  sedentary  soils,  having  been  formed  from  the  immedi- 
ately underlying  rocks,  vary  in  character  from  place  to  place, 
and  here  and  there  the  slopes  are  so  steep  that  little  or  no  soil 
can  accuniulate.  The  soils  derived  from  the  Cretaceous  and 
Eocene  formations  are  perhaps  those  best  adapted  to  agricul- 
ture, but  they  lie  chiefly  on  high  ground  that  is  cut  by  rather 
steep  canyons,  so  that  they  are  not  so  generally  cultivated  as 
the  soils  of  the  lower  ground.  The  sandstones  of  the  Monte- 
rey group,  which  are  very  quartzose,  yield  nearly  everywhere  a 
light  sandy  soil,  whereas  the  shale  and  chert  formations  of  the 
same  group  yield  scant  and  poor  soils.  Considerable  areas 
that  are  underlain  by  the  shale  and  chert  carry  no  soil  what- 


14 


ever,  the  bare  mechanically  disintegrated  rock  forming  the  sur- 
face of  the  ground.  The  fresh-water  deposits  of  the  Orinda 
formation  have  generally  yielded  deep  and  excellent  soils, 
which,  however,  are  in  man}^  places  heavy  and  clayey. 

The  soil  in  the  bottoms  of  the  vallej'^s  is  not  sedentary  but 
has  been  derived  from  various  sources  in  the  course  of  the 
degradation  of  the  surrounding  hills  and  is  excellent.  It  varies 
from  a  sandy  to  a  clayey  loam  and  in  certain  localities  is  even 
gravelly,  but  in  practically  all  places  it  is  well  adapted  to  suc- 
cessful tillage. 

Vegetation. — The  region  is  almost  devoid  of  forest.  Hill- 
tops and  slopes  are  bare  of  trees  (see  PI.  II)  and  the  prevail- 
ing mantle  of  vegetation  in  uncultivated  tracts  is  composed 
of  the  wild  oat  {Aveiia  fatua)  and  other  wild  grasses  {Danthonia 
californica,  Festuca  myuros,  Lolium  temulentum,  and  species 
of  Hardeum);  but  on  the  south  sides  of  steep  canyons  the 
slopes  may  be  covered  with  a  more  or  less  dense  growth  of 
brush.  The  only  native  timber  to  which  the  term  forest 
might  be  applied  is  the  grove  of  redwoods  (Sequoia  semper- 
virens)  on  the  west  side  of  Redwood  Canyon,  extending  from 
the  creek  to  the  summit  of  Redwood  Peak.  The  only  other 
conifers  in  the  district  are  the  digger  pine  {Pinus  sabiniana), 
which  is  a  feature  of  the  lower  slopes  of  Mount  Diablo  on 
the  east  side  of  San  Ramon  Valley,  and  the  knobcone  pine 
(Pinus  alter iiata),  which  grows  locally  on  the  summit  of  the 
first  ridge  east  of  Redwood  Canyon. 

The  most  abundant  tree  is  the  live  oak  (Quercus  agrifolia), 
common  in  canyons  and  on  north  and  east  slopes  and  also 
notable  for  filling  shallow  gulches  or  south  and  west  slopes  of 
otherwise  treeless  hills.  The  valley  oak  (Quercus  lobata)  is 
characteristic  of  the  open  valleys,  growing  onl}'^  sparingl}'^  or 
not  at  all  on  the  hills.  It  is  most  common  in  San  Ramon  and 
Ygnacio  valleys.  The  blue  oak  (Quercus  douglasii)  is  scattered 
over  dry  hills  in  the  eastern  part  of  the  area.  The  buckeye 
(JEsculus  californica)  grows  along  the  bases  of  low  hills.  The 
laurel  (Umbellularia  californica)  is  the  commonest  tree  in  can- 
yon bottoms.  It  also  clusters  about  rocky  knolls  on  ridges  and 
slopes.     Along  the  stream   courses  are   found   the   red    alder 


15 

[Alnus  oregona)  and  the  white  alder  (Alnus  rhomb  if olia),  the 
latter,  however,  only  east  of  the  crest  of  the  Berkeley  Hills. 
There  are  three  willows,  the  most  abundant  and  widely  distrib- 
uted being  the  white  willow  or  arroyo  willow  (Salix  lasiolepis), 
which  thrives  in  dry  gulches  in  the  hills  as  well  as  along  living 
streams.  The  yellow  willow  (Salix  lasiayidra)  and  the  red  wil- 
low [Salix  Iwvigata)  are  mostly  confined  to  living  streams.  The 
madrone  [ArhiLtun  menziesii)  is  rare  and  is  found  chiefly  with  the 
redwoods  but  extends  northward  to  Strawberry  Canyon.  The 
big-leaf  maple  {Acer  macrophyllum),  the  box  elder  {Negundo 
caUformcwu),  and  the  sycamore  {Platanus  racemosa)  are  rare. 
The  California  walnut  [Juglans  californica)  grows  along 
streams  at  Walnut  Creek  and  Lafayette.  Taking  the  territory 
as  a  whole,  the  brush-covered  areas  are  more  striking  than  the 
wooded  areas.  The  brush  of  the  Berkeley  Hills  is  composed 
chiefly  of  nine-bark  [Neillia  capitata),  cofl*ee  berry  [Rhamnus 
californica),  hill  scrub  [Baccharis  pilularis),  poison  oak  (Bhus 
diversiloba),  and  mountain  lilac  {Ceanothus  thyrsiflorus  and 
C.  sorediatus).  Typical  chaparral  is  not  common,  although 
colonies  of  manzanita  {Arctostaphylos  manzanita)  and  similar 
shrubs  are  found,  particularly  east  of  the  crest  of  the  Berkeley 
Hills.  Chamiso  grows  more  or  less  extensively  on  Las  Tranipas 
Ridge,  on  the  hill  east  of  Bed  wood  Peak,  and  at  the  base  of 
Mount  Diablo." 

Climate. — The  crest  of  the  Berkeley  Hills  is  a  dividing  line 
for  the  climate  of  this  region.  The  climate  of  the  area  east  of 
the  crest  is  somewhat  like  that  of  the  interior  valleys;  that 
of  the  west  slope  of  the  Berkeley  Hills  is  like  that  of  the  coast. 
In  the  area  east  of  the  crest  the  summers  are  hotter  and  the 
winters  are  colder  than  in  the  area  farther  west.  The  sea 
breezes  and  fogs  that  temper  the  summer  heat  on  the  west 
slope  have  a  greatly  diminished  influence  on  the  east  side  of 
the  Berkeley  Hills.  The  annual  rainfall  at  Berkeley  is  about 
27.48  inches,  and  it  falls  almost  wholly  in  winter,  the  summer 
being  rainless.  Snow  rarely  falls,  even  on  the  highest  ground, 
and  no  snowfall  lasts  more  than  a  day. 

"Furiijucli  of  the  information  contained  in  thiss  and  otiier  paragraphs 
dealing  with  the  vegetation  the  writer  is  indebted  to  Prof.  W.  L.  Jepson. 


16 


SAN    FRANCISCO    PENINSULA. 


The  territory  west  of  the  Bay  of  San  Francisco  is  naturally 
divided  into  a  northern  and  a  southern  part  by  the  Golden 
Gate.  South  of  the  Golden  Gate  lies  the  San  Francisco  Penin- 
sula, with  the  city  of  San  Francisco  at  its  northern  extremity; 
north  of  it  lies  the  Marin  Peninsula,  w^hose  most  notable  fea- 
ture is  Mount  Tamalpais. 

Relief  and  drainage. — The  San  Francisco  Peninsula  is  divided 
into  two  parts  by  Merced  Valley.  Each  of  these  parts  has  the 
general  profile  of  a  much  dissected  orographic  block  having  a 
gentle  slope  to  the  northeast -and  a  crest  line  on  its  southwest 
margin.  The  culminating  crest  of  the  northern  block  is  San 
Bruno  Mountain  (elevation  1315  feet),  and  the  corresponding- 
crest  of  the  southern  block  is  Montara  Mountain  (elevation 
1952  feet).  The  structural  line  separating  the  surface  areas  of 
the  blocks  is  the  trace  of  the  San  Bruno  fault,  which  lies  at 
the  base  of  the  steep  southwest  front  of  San  Bruno  Mountain. 
In  character  of  relief  these  two  blocks  present  an  interesting- 
contrast.  The  surface  of  the  northern  block  is  irregularly 
hilly  and,  except  in  San  Bruno  Mountain,  shows  but  little 
linearity  in  the  disposition  of  its  crests  and  valleys.  Its  geo- 
inorphy  is  fairly  mature,  although  a  characteristic  feature  of 
that  maturity  is  a  certain  ruggedness  of  profile  due  to  the 
presence  of  formations  composed  of  radiolarian  chert.  The 
exceptional  resistance  which  these  offer  to  erosion  causes  the 
areas  they  occupy  to  present  a  marked  contrast  with  adjoining 
areas  composed  of  sandstone.  San  Bruno  Mountain,  however, 
is  a  simple  linear  ridge  with  an  immature  frontal  slope  over- 
looking Merced  Valley. 

The  southern  block  is  marked  by  notable  linearity  in  its 
crests  and  valleys,  and  its  geomorphy  is  in  general  much  less 
mature  than  that  of  the  northern  block.  The  most  remarkably 
linear  feature  is  the  valley  in  which  lie  San  Andreas  and  Crys- 
tal Springs  lakes  (see  PI.  IX),  a  short  segment  of  the  San 
Andreas  rift  valley,  a  feature  due  to  repeated  faulting  in  recent 
geologic  time.  The  trace  of  the  fault  of  1906  follows  this  rift 
for  about  300  miles.  The  valley  is  due  in  part  directly  to 
earth    movement    but  in   large    measure  also    to  the  mashing 


17 


of  the  rock  in  the  fault  zone  and  its  consequent  easy  erosion. 
Northeast  of  the  San  Andreas  rift  lies  the  equally  linear 
Buril)uri  Ridge,  which  slopes  down  to  Merced  Valley  and 
to  the  Bay  of  San  Francisco.  San  Mateo  (^reek  cuts  across 
this  ridge  transversely  at  nearly  its  widest  part,  draining 
the  valley  of  the  San  Andreas  rift,  both  tlie  part  which 
lies  northwest  and  that  which  lies  southeast  of  the  point 
where  the  creek  leaves  the  rift  to  flow  through  a  sharply 
incised  gorge  toward  the  Bay  of  San  Francisco.  The  position 
of  this  trunk  drainageway  of  the  rift  valley  indicates  that  it  is 
a  superimposed  stream,  the  course  of  which  was  established 
when  the  slope  was  mantled  by  softer  formations,  now 
removed.  The  slope  of  the  Montara  orographic  block  in  the 
area  southwest  of  the  rift  valley  is  dissected  by  a  series  of  sub- 
parallel  steep-sided  canyons  and  intervening  ridges,  which  are 
in  large  part  remnants  of  the  original  tilted  surface.  Viewed 
in  a  large  way,  however,  this  tilted  slope,  taken  as  a  wdiole, 
from  the  crest  of  Montara  Mountain  to  the  Bay  of  San  Fran- 
cisco, presents  a  broadly  terraced  aspect.  It  comprises  two  ter- 
race levels  or  steps,  one  the  flat-topped  ridge  that  is  elsewhere 
called  the  Buriburi  Plateau,  the  general  elevation  of  which  is 
about  700  feet,  and  the  other  the  Sawyer  Plateau,  comprising 
a  number  of  flat-topped  ridges  that  stand  at  elevations  of  1100 
to  1200  feet. 

The  crest  of  the  tilted  block  known  as  Montara  Mountain  is 
composed  of  quartz  diorite  and  is  somewhat  serrate  in  its  longi- 
tudinal profile,  its  serration  being  due  to  its  incision  by  the  head- 
water erosion  of  the  high-grade  streams  on  its  southwest  front. 
On  the  precipitous  shoulders  and  ridges  of  the  southwest  face 
of  the  mountain  there  are  obscure  traces  of  terraces  at  elevations 
of  400  to  500  feet.  At  the  base  of  the  mountain  front,  but 
separated  from  it  by  an  alluviated  valley,  is  a  low  ridge,  known 
as  Miramontes,  terminating  in  Pillar  Point.  This  ridge  is  com- 
posed of  folded  late  Pliocene  strata,  which  rest  upon  the  quartz 
diorite.  A  little  north  of  this,  at  Montara  Point,  is  a  fine 
example  of  a  superimposed  stream  cutting  through  a  foothill 
ridge,  with  lower  ground  between  it  and  the  base  of  Montara 
Mountain. 

San  Francisco— 2 


18 


The  sand  dunes  of  the  city  of  San  Francisco  are  notable 
features  of  the  north  end  of  the  peninsula.  The  sand  is  blown 
in  from  the  beach  south  of  the  Golden  Gate  by  the  westerly 
winds  and  drifts  eastward  over  the  western  part  of  the  city  as  a 
wide  expanse  of  ripple-marked  dunes.  The  encroachment  of  the 
sand  is  now,  however,  in  large  measure  checked  by  the  extension 
of  city  improvements  toward  the  beach  and  by  the  planting  of 
suitable  vegetation  in  the  sand  to  restrain  its  movement. 

Shore  lines. — The  shore  lines  of  the  two  sides  of  the  peninsula 
present  a  marked  contrast.  On  the  bay  side  the  shore  contour 
is  im maturely  serrate,  with  tidal  marshes  and  other  evidences  of 
silting  in  the  bays  between  the  points,  cliff  cutting  is  relatively 
feeble,  and  there  are  no  clean  sandy  or  pebbly  beaches.  On 
the  ocean  side  the  attack  of  the  waves  has  developed  extended 
lines  of  sea  cliffs  with  sandy  beaches  at  their  bases,  and  the 
embayments  between  the  cliffs  are  filled  out.  This  process  has 
proceeded  so  far  as  to  give  the  shore  a  simple  and  mature  con- 
tour, only  a  few  short  points — as,  for  example,  San  Pedro 
Point — projecting  seaward.  The  shore  features,  particularly 
those  on  the  bay  side  of  the  peninsula,  indicate  the  recent  sub- 
sidence of  an  erosionally  dissected  land  mass,  and  a  like  subsi- 
dence on  the  seaward  side  is  indicated  by  Merced  Lake,  which 
before  it  was  modified  for  use  as  a  reservoir  was  a  flooded 
stream  valley  shut  off  by  drifting  sands. 

Soil. — The  soil  of  the  peninsula  is  largely  sedentary  on  the 
gentler  slopes  of  the  surfeee  that  are  underlain  by  the  sandstones 
of  the  Franciscan  group  and  the  comparatively  soft  strata  of 
the  Merced  (Pliocene)  and  Quaternary  formations.  But  little 
soil  accumulates  upon  areas  occupied  by  the  radiolarian  cherts 
of  the  Franciscan  group,  by  the  serpentine  associated  with  the 
Franciscan,  or  by  the  quartz  diorite  ("Montara  granite").  At 
the  base  of  the  hills  and  in  some  of  the  valleys  there  are  belts 
of  rich  soil  formed  by  the  accumulation  of  alluvium  washed 
from  higher  levels.  In  the  vicinity  of  San  Francisco  the  loose 
sandy  soil  is  intensively  cultivated,  with  the  liberal  use  of  fertil- 
izers, for  growing  vegetables  for  the  local  market.  A  part  of 
the  hill  laud  is  adapted  to  cattle  grazing  and  dairying,  but  the 
large  reserves  maintained  by  the  Spring  Valley  Water  Co.  to 


19 


keep  the  city  water  supply  free  from  pollution  have  restricted 
the  areas  available  for  these  industries. 

Veyelation. — Climatic  conditions  are  adverse  to  forest  growth 
on  the  San  Francisco  Peninsula.  Most  of  the  high  ground  is 
bare  of  trees.  In  the  gullies  and  canyons,  however,  there  is  a 
more  or  less  luxuriant  growth  of  shrubs  and  low  trees,  compris- 
ing California  laurel  (UmbeUidaria  californica),  coffee  berry 
(Rhamnus  californica),  creek  dogwood  (Corniis  piibescens), 
arroyo  willow  [Salix  lasiolepis  var.  bigelovii),  wax  myrtle 
(Mi/rica  californica),  coast  live  oak  [Quercus  agrifolia),  and 
blue  blossom  (Ceanothus  thyrsijiorus). 

On  the  open  hills  may  be  found  the  following  common  and 
widely  distributed  species  of  shrubs:  Old  man  [Artemisia  cali- 
fornica), bush  monkey  flower  (Diplacus  ghitinosus),  woolly 
painted  cup  {Castilleia  foliolosa),  JSricameria  ericoides,  eoifee 
berry  [Rhamnus  californica),  Christmas  berry  {Hetero7ncles 
arbutifolia),  poison  oak  {Rhus  diversiloba),  and  bush  lupine 
(Lupiniis  arboreus). 

The  following  are  the  more  common  plants  that  grow  on  the 
sand  dunes  and  tend  to  restrain  their  movement:  (Enothera 
cheiranthifolia,  coyote  scrub  [Baccharis  doiiglasii),  sea  fig 
{3Iesembryanihemum  mqui  later  ale)  (introduced),  dune  tansy 
(Tanacetum  camphoratum),  Franseria  chamissonis,  Roubieva 
midtifida,  Franseria  bipinnatifida,  Crotou  californicus,  Polygo- 
71U771  paronychia,  sand  grass  (Poa  douglasii),  and  beach  grass 
(Ammophila  and  Arenaria,  introduced). 

On  the  sea  cliffs  may  be  found  a  flora  peculiar  to  these  steep 
and  exposed  slopes,  comprising  the  sea  daisy  [Erigeron  glaucus), 
lizard  tail  (Friophylhim  stwchadifolium),  yarrow  (Achillwa 
millefolium),  seaside  painted  cup  (Castilleia  latifolia),  and  Eri- 
gonum  latifolium.  A  few  plants  are  peculiar  to  the  beaches, 
such  as  the  sea  plantain  (Plantago  maritima),  sand  strawberry 
(Fragaria  chilcnsis),  and  beach  pea  [Lathyrus  littoralis). 

The  vegetation  of  the  salt  marshes  comprises  Salicomia 
ambigua,  Triglochin  maritima,  Frankenia  grandifolia,  Atriplcx 
patula,  Atriplex  hastata,  Tissa  macrotheca,  and  Cotula  coronopi- 
folia.  Near  the  tidal  channels  and  in  the  parts  of  the  marshes 
that  liave  been  longest  reclaimed  to  vegetation  there  may  be 


20 

found  a  variety  of  sedges  and  tules,  such  as  Scirpus  lacustris 
var.  occidentalis,  Scirpus  olneyi,  Scirpus  robushts,  Cyperus  bron- 
gniartii,  Juncus  jjatens,  Juncus  xip/iioides,  and  Typha  latifoUa. 
Climate. — At  San  Francisco  the  only  marked  seasonal  change 
is  due  to  difference  in  precipitation.  The  mean  annual  temper- 
ature is  56°  F.  The  coldest  month  is  January,  which  has  a 
mean  temperature  of  50°,  and  the  warmest  is  the  period  from 
the  middle  of  September  to  the  middle  of  October,  during 
which  the  mean  temperature  is  60°.  On  summer  afternoons  a 
layer  of  fog  1700  feet  deep  spreads  over  -the  city.  The  mean 
annual  rainfall  is  22.83  inches,  which  falls  in  winter,  the  sum- 
mer being  rainless.  The  prevailing  winds  are  westerly,  and 
their  average  velocity  is  9.7  miles  an  hour.  The  mean  relative 
humidity  is  88  per  cent  in  the  morning  and  73  per  cent  in  the 
evening.  At  Pilarcitos  Lake,  about  15  miles  south  of  the  center 
of  San  Francisco,  the  annual  rainfall  is  about  double  that  at  the 
city;  but  the  rainfall  decreases  southeastward,  along  the  valley 
of  the  bay,  from  22.83  inches  at  San  Francisco  to  15  inches  at 
San  Jose.  Places  on  the  bay  side  of  the  peninsula  are  colder 
in  winter  and  warmer  in  summer  than  the  city  of  San  Francisco. 
In  winter  the  valley  bottoms  on  the  bay  side  are  subject  to 
frequent  morning  frosts,  but  on  the  slopes  of  the  surrounding 
hills  frosts  are  less  frequent  and  less  severe.  In  summer  the 
valleys  may  be  bathed  in  bright  sunshine  while  San  Francisco 
and  the  Golden  Gate  are  mantled  in  dense  fosr.  The  climate 
of  the  peninsula  thus  presents  marked  local  variations,  and 
these  variations  are  doubtless  related  to  the  relief. 

MARIN    PENINSULA. 

Relief. — The  Marin  Peninsula  presents  the  features  of  a  dis- 
sected mountain  mass  which  has  been  depressed  sufficiently  to 
permit  the  waters  of  the  ocean  to  enter  the  stream  valleys  and 
so  convert  them  into  bays  or  inlets.  Its  highest  point  is  Mount 
Tamalpais,  which  has  an  elevation  of  2604  feet.  From  this 
peak  a  ridge  with  a  fairly  even  crest  extends  westward  and 
then  northwestward  beyond  the  Tamalpais  quadrangle.  From 
the  peak  and  crest  the  surface  falls  away  in  steep  slopes  on  all 
sides,   in  an  alternation  of  canyons  and  steep-crested   ridges. 


21 


The  canyons  are  all  of  steep  grade  and  have  steep  sides,  but 
where  they  reach  the  sea  level  they  widen  very  notably  and 
their  bottoms  become  flaring,  flat  valley  floors,  which  are 
occupied  for  the  most  part  by  salt  marshes.  This  description 
applies  particularly  to  the  bay  side  of  the  peninsula,  where 
Tiburon  Peninsula  and  San  Quentin  Point  separate  three 
embayments  that  reach  far  inhmd  to  the  base  of  the  mountain 
and  that  are  very  evidently  drowned  valleys.  On  its  w^est  side 
the  main  mass  of  the  Marin  Peninsula  has  a  very  even  steep 
slope,  which  in  part  descends  to  the  sea  and  in  part  to  the 
bottom  of  the  straight,  narrow  valley  that  separates  the  Point 
Peyes  Peninsula  from  the  mainland.  This  declivity  ranges  in 
width  from  1^  to  2  miles  and  except  at  its  southern  end  is  not 
deeply  trenched  by  the  streams  that  cross  it.  This  western 
shore  of  the  Marin  Peninsula,  which  is  without  notable  embay- 
ments and  promontories,  presents  a  marked  contrast  to  the 
extremely  indentate  and  irregular  eastern  shore  on  the  bay 
side,  and  the  geomorphic  asymmetry  of  the  peninsula  suggests 
that  it  is  a  tilted  orographic  block,  elevated  on  its  western  and 
depressed  on  its  eastern  margin.  This  interpretation  of  the 
origin  of  its  geomorphic  features  agrees  with  that  given  for  the 
north  end  of  the  San  Francisco  Peninsula,  for,  as  the  west  or 
southwest  boundary  of  the  Marin  block  is  similar  to  the  south- 
west boundary  of  the  San  Francisco  block,  as  exemplified  in 
the  front  of  San  Bruno  Mountain,  and  as  the  two  lie  in  the 
same  general  straight  line,  the  mainland  of  the  Marin  Penin- 
sula and  the  north  end  of  the  San  Francisco  Peninsula  are 
evidently  parts  of  the  same  tilted  crustal  block  and  have 
had  a  common  geomorphic  history.  This  block  is  transected 
by  the  Golden  Gate,  which  separates  it  superficially  into 
two  parts.  The  Golden  Gate  is  without  doubt  the  gorge  of 
a  trunk  stream  which  maintained  itself  across  the  block  dur- 
ing the  slow  progress  of  its  tilting  and  which  subsequently, 
after  the  depression  of  the  region,  became  a  marine  strait 
through  which  the  sea  flooded  the  valley  on  the  lower  side  of 
the  block. 

Besides  the  Golden  Gate,  another  valley  completely  transects 
the  tilted  block.     This  is  Ellk  Valley,  which  extends  across  the 


0'7 


]\I;irin  Peninsula  from  the  head  of  Richardson  Bay  to  Tennessee 
Cove.  It  is  a  narrow  valley  of  low  gradient,  which  has  steep 
mountain  slopes  on  both  sides  and  which  lies  at  right  angles  to 
the  axis  of  the  block.  The  bottom  of  this  valley,  though  nar- 
row, is  alluviated  throughout  by  the  wash  from  the  adjacent 
slopes.  The  highest  point  of  the  valley  bottom  is  about  mid- 
way between  the  bay  and  the  ocean  and  lies  between  the 
175-foot  and  200-foot  contours.  This  point  is  evidently  the 
headwater  portion  of  a  stream  that  crossed  a  part  of  the  block 
prior  to  the  tilting.  Rodeo  Lagoon,  just  north  of  the  Golden 
Gate,  occupies  the  only  drowned  valley  on  the  west  side  of  the 
mainland  of  the  Marin  Peninsula.. 

San  Andreas  rift  valley. — The  Point  Reyes  Peninsula  is  geo- 
graphically distinct  from  the  mainland  of  the  Marin  Peninsula 
and  is  separated  from  it  by  a  long,  narrow,  straight  valley,  the 
north  end  of  which  is  occupied  by  Tomales  Bay  and  the  south 
end  by  Boliuas  Lagoon,  a  lake  cut  off  from  the  ocean  by  a 
sandspit.  This  forms  a  notable  segment  of  the  San  Andreas 
rift  valley,  and  the  trace  of  the  fault  of  April  18,  190G,  runs 
completely  through  it.  Its  straight,  linear  course  was  undoubt- 
edly determined  by  the  existence  of  the  zone  of  recurrent  fault- 
ing which,  partly  by  displacement  and  partly  by  excessive  ero- 
sion induced  by  rock  mashing,  finds  topographic  expression  in 
the  rift  valley. 

Point  Reyes  Peninsula. — The  dominant  feature  of  the  relief 
of  the  Point  Reyes  Peninsula  is  a  comparatively  straight  ridge 
on  its  eastern  margin,  parallel  to  and  close  to  the  rift  valley. 
From  this  ridge  the  ground  slopes,  w^itli  many  minor  irregu- 
larities, westward  to  the  seashore.  The  streams  that  drain  this 
slope  have  in  their  lower  stretches  been  drowned  by  subsi- 
dence, and  both  Tomales  Bay  and  Bolinas  Lagoon  were  also 
formed  by  subsidence. 

At  the  south  end  of  the  Point  Reyes  Peninsula  is  one  of  the 
most  clearly  evident  wave-cut  terraces  on  the  coast  of  Cali- 
fornia. It  has  the  form  of  a  very  even  topped,  gently  sloping 
plateau,  the  rear  of  which,  where  it  abuts  against  ancient  and 
now  much  degraded  sea  cliffs,  stands  about  250  feet  above  the 
sea.     This  wave-cut  terrace  has  a  maximum  width  of  over  a 


23 


mile  and  u  half  and  terminates  on  its  seaward  side  at  the  brink 
of  the  modern  sea  cliffs.  It  is  now  dissected  by  numerous 
small  streams,  which  run  seaward  across  it  from  the  high 
ground  on  the  north. 

We  thus  have  side  by  side — in  the  drowned  streams  and  the 
elevated  sea  cliifs — abundant  evidence  of  the  opposite  move- 
ments of  elevation  and  subsidence  of  this  part  of  the  coast, 
subsidence  having  been  probably  the  later  movement.  It  is 
remarkable  that  although  there  is  so  fine  an  elevated  wave-cut 
terrace  on  the  Point  Reyes  Peninsula  there  is  no  well-defined 
trace  of  elevated  strands  on  the  western  shore  of  the  mainland 
of  the  Marin  Peninsula. 

Soil. — The  soil  of  the  Marin  Peninsula  is  chiefly  sedentary, 
so  that  its  character  in  most  places  depends  on  that  of  the 
underlying  formations.  The  principal  formation,  the  sand- 
stone of  the  Franciscan  group,  yields  a  sandy  loam  soil,  which, 
however,  is  scant  on  the  prevailingly  steep  slopes.  There  is 
but  little  soil  upon  the  radiolarian  chert  formations  of  the 
Franciscan  group  and  the  associated  serpentine.  The  splie- 
roidal  basalt  yields  a  red  soil,  which  is  deep  enough  to  be  till- 
able on  gentle  slopes.  In  the  valley  bottoms  above  the  limit 
of  the  salt  marsh  the  soil  is  a  fertile  alluvium  composed  of  the 
wash  of  the  valley  slopes.  Cattle  raising  and  dairying  are  the 
chief  agricultural  industries  on  the  peninsula. 

Vegetation. — The  woody  vegetation  of  the  Marin  Peninsula 
is  either  chaparral  or  tree  growth  of  various  sorts.  Chaparral 
is  tlie  dominant  type.  It  cov^ers  the  main  slopes  of  Mount 
Tamalpais  up  to  its  summit  and  considerable  areas  farther 
north,  along  Bolinas  Ridge  and  toward  San  Rafael,  becoming 
less  abundant  toward  the  north.  It  includes  mainly  the  fol- 
lowing species:  Arctostaphylos  tomentosa,  A.  manzanita,  A. 
nummular ia,  Cennothus  thyrsifiorus,  C.  foliosus,  C.  rlgidus, 
C.  prostratus  var.  divergens,  C.  cuneatus,  C.  soredlatus,  Garrya 
elliptica,  Rhamnus  californica,  Quercus  duniosa,  Q.  wislizenii 
ynr.  frutescens,  and  Adenostoma  fasciculatum,  or  chamiso. 

The  tree  growth  is  confined  mostly  to  canyons  or  occurs  as 
a  scattered  stand  on  northern  slopes.  In  tlie  canyons  it  con- 
sists mainly  of  Sequoia  sempervirens,  Pseudotsuya  taxifolia,  and 


24 


Umbellularia  californica.  On  the  lower  northern  slopes  of 
Mount  Tamalpais  is  an  open  stand  comprising-  Quercus  cali- 
fornica (the  most  abundant  species),  Q.  garryana,  and  Arbutus 
menziesii. 

Along  the  streams  may  be  found  Fraxinus  oregana,  Alnus 
rubra,  and  Acer  macroiphyUum.  Associated  with  the  scattered 
oaks  there  is  considerable  RIiub  diversiloba,  and  the  redwoods 
of  the  canyons  are  accompanied  by  the  plants  that  commonly 
form  the  undergrowth  of  the  main  redwood  belt,  comprising 
Vaccinium  ovatum,  Gaultheria  shallon,  Oxalis  oregana,  Scoli- 
opus  bigelovii,  Trillium  ovatum,  and  Clintonia  andrewsiana. 

On  the  western  slope  of  Mount  Tamalpais  are  a  few  patches 
of  the  rare  Cupressus  sargentii,  and  in  the  valleys  near  San 
Rafoel  is  a  form  of  valley  oak  that  shuns  the  coast. 

The  vegetation  on  the  west  side  of  the  San  Andreas  rift 
valley  is  radically  different  from  that  on  the  east  side.  From 
Bo  Unas  Lagoon  northward  the  eastern  slope  of  the  main  ridge 
of  the  Point  Reyes  Peninsula  is  covered  with  a  forest  which, 
though  not  continuous,  is  fairly  dense  in  the  areas  where  it 
is  best  developed.  This  forest  is  composed  almost  exclusively 
of  Pinus  muricata,  which  is  accompanied  by  a  little  Pasania 
densijiora  and  Quercus  agrifolia  and  by  considerable  Umbel- 
lularia californica  on  very  steep  slopes.  The  densest  part 
of  the  forest  is,  however,  pure  Pinus  muricata.  The  shrubs  of 
the  Point  Keyes  Peninsula  are  northern  types,  which  have 
here  their  southernmost  or  nearly  their  southernmost  repre- 
sentation. These  shrubs  include  Rubus  spectabilis  var.  men- 
ziesii,  Ledum  glandulosum,  and  Rhododendron  callfornicum. 

The  vegetation  of  the  marsh  lands  is  practically  the  same  as 
that  which  characterizes  the  marshes  of  the  San  Francisco 
Peninsula,  already  listed. 

Climate. — The  climate  of  the  Marin  Peninsula,  unlike  that 
of  the  city  of  San  Francisco,  is  characterized  by  foirly  well 
marked  seasonal  changes  in  temperature.  The  mean  annual 
temperature  is  dfy^  F.,  nearly  the  same  as  that  of  San  Fran- 
cisco, but  the  mean  January  temperature  is  40''  and  the  mean 
July  temperature  is  70\  The  mean  annual  rainfall  at  Kent- 
field  (elevation  Go  feet)  is  ol.34  inches,  which  is  over  double 


25 

that  of  San  Francisco;  but  at  Point  Reyes  Light  it  is  only 
30.80  inches.  Tlie  prevailing  winds  are  northwest,  and  their 
average  velocity  on  Mount  Tainalpais  is  17.8  miles  an  hour. 
The  maximum  velocities  occur  at  Point  Reyes  Light.  The 
summer  afternoon  fogs  generally  do  not  extend  to  the  top  of 
Mount  Tamalpais,  and  the  upper  surface  of  the  fog  seen  in  the 
bright  sunlight  from  the  summit  is  a  most  remarkable  and 
beautiful  sight.  The  coast  north  of  the  Golden  Gate,  particu- 
larly in  summer,  is  covered  by  a  sea  fog  100  to  1700  feet  thick, 
which  lies  along  the  coast  wdth  its  bottom  frequently  a  hundred 
feet  or  less  above  sea  level. 

BAY    OF    SAN    FRANCISCO. 

The  Bay  of  San  Francisco  is  a  submerged  valley  and  is  a 
most  notable  example  of  a  great  harbor  formed  by  the  influx  of 
the  sea  into  the  low  parts  of  a  subsiding  coast.  (See  PI.  IV.) 
If  the  region  were  uplifted  so  that  the  water  were  drained 
out  of  the  bay,  the  depression  would  not  differ  in  its  essen- 
tial features  from  the  Santa  Clara,  Santa  Rosa,  or  Napa  val- 
leys. If,  on  the  other  hand,  the  coast  were  still  farther 
depressed,  so  that  these  valleys  also  were  flooded,  they  would 
have  features  entirely  analogous  to  those  of  the  Bay  of  San 
Francisco.  The  isolated  hills  and  some  of  the  foothill  ridges 
would  become  islands  similar  to  Angel  Island  and  Goat  Island, 
the  tributary  valleys  would  become  embayments  or  inlets  simi- 
lar to  Richardson  Bay  and  San  Rafael  Bay,  and  the  inter- 
vening ridges  would  become  peninsulas  or  promontories  like 
Tiburon  Peninsula,  Hunter  Point,  and  San  Pablo  Point. 

Before  the  valley  of  the  Bay  of  San  Francisco  was  submerged 
there  flowed  through  it  a  river  that  drained  the  great  interior 
valley  of  California.  This  river  probably  ran  between  the 
Tiburon  Peninsula  and  Angel  Island,  where,  in  Raccoon  Strait, 
there  is  a  deep  channel  witli  a  sounding  of  39  fathoms. 
Thence  it  flowed  through  the  gorge  of  the  Golden  Gate,  where 
the  present  maximum  depth  of  water  is  69  fathoms,  between 
Fort  Point  and  Lime  Point.  The  position  of  this  ancient 
river  in  Raccoon  Strait  suggests  that  it  w^as  a  superimposed 
stream  whose  course  had  been  determined  when  the  region  was 


26 


covered  with  soft  Pliocene  formations,  which  have  since  been 
carried  away  by  erosion. 

The  waters  of  the  bay  that  lie  away  from  the  main  channel  are 
comparatively  shallow.  From  Raccoon  Strait  to  San  Pedro 
Point  the  main  channel  has  a  maximnm  depth  of  16  fathoms 
in  the  narrowest  place,  but  the  depth  of  the  water  in  general  in 
this  part  of  the  bay  ranges  from  4  to  8  fathoms.  The  channel 
through  San  Pablo  Bay  at  most  places  beyond  San  Pedro  Point 
does  not  exceed  4  fathoms  in  depth,  but  it  deepens  notably 
at  Carquinez  Strait,  where  it  is  constricted.  The  water  in 
San  Pablo  Bay  is  shallow,  its  depth  averaging  perhaps  7  feet 
at  low  tide.  In  the  area  between  San  Francisco  and  Goat 
Island  the  maximum  depth  is  20  fathoms,  and  south  of  this 
area  the  deeper  water  channel  has  in  general  a  depth  diminish- 
ing from  10  fathoms  to  6  fathoms,  with  rather  shallow  water 
on  both  sides.  The  fact  that  the  deepest  channel  lies  in  the 
most  constricted  parts  indicates  that  these  depths  are  maintained 
by  tidal  scour  and  that  the  present  deep-water  channel  can  not 
be  assumed  to  represent  throughout  its  course  the  trench  of  the 
ancient  river. 

Before  the  submergence  of  the  valley  now  occupied  by  the 
bay  the  streams  draining  the  surrounding  hills  flowed  clown 
over  more  or  less  gravelly  or  sandy  bottoms  to  the  trunk  drain- 
ageway  and  spread  gravel  and  sand  over  the  valley  bottom. 
As  subsidence  proceeded  these  gravels  and  sands  became  buried 
by  finer  silts.  This  process  continued  by  stages,  so  that 
beneath  the  floor  of  the  valley  and  beneath  the  bay  there  is  an 
alternation  of  sands  or  gravels  with  finer  silts  or  clays,  and 
some  of  these  deposits  contain  marine  fossils,  which  have 
been  discovered  bv  borins;.  This  recurrent  burial  of  fluviatile 
gravels  and  sands  by  fine  silt  or  clay  involved  many  changes 
in  the  courses  of  the  stream  channels  as  they  ran  out  from  the 
canyons  over  the  floor  of  the  valley,  but  each  of  these  gravelly 
channels,  wliatever  its  course  at  any  stage  of  the  infilling  of 
the  valley,  remained  connected  as  a  strip  of  sand  and  gravel 
with  the  portion  of  the  stream  that  lay  above  the  zone  of  aggra- 
dation.    Thus  were  established  the  conditions  of  an  artesian 


11 


basin,  the  features  of  which  will  be  further  described  under  the 
headino-"  Economic  oeolo2;v." 

The  Bay  of  San  Francisco  is  evidently  a  submerged  land 
valley,  and  the  geomorphic  features  of  its  periphery  and  the 
valley  itself  were  manifestly  shaped  by  the  ordinary  agencies 
of  erosion,  but  as  tlie  valley  lies  on  the  relatively  depressed 
sides  of  the  San  Francisco-Marin  and  Montara  crustal  blocks 
it  was  probably  in  large  part  outlined  by  the  movement 
that  tilted  these  blocks,  and  to  that  extent  it  is  of  diastrophic 
origin.  A  second  movement  of  depression  appears  to  have 
affected  the  region  as  a  whole  and  allowed  the  sea  to  enter  the 
gorge  at  the  Golden  Gate,  which  had  been  cut  by  stream 
erosion  across  the  crest  of  the  rising  side  of  the  northern  block. 
That  the  earth  movements  in  this  region  were  not  simple  is 
shown  by  the  fact  that  on  the  southeast  side  of  San  Pablo  Bay 
and  about  the  west  end  of  Carquinez  Strait  there  are  wave-cut 
terraces  and  elevated  deposits  of  marine  shells  of  species  that 
are  still  living ;  whereas  in  the  area  south  of  San  Pablo  Bay 
there  are  no  such  terraces  or  elevated  late  Quaternary  marine 
deposits.  The  evidences  of  uplift  on  San  Pablo  Bay  appear 
on  a  third  and  quite  distinct  crustal  block,  represented  by  the 
Berkeley  Hills,  and  the  only  place  where  this  block  touches 
the  Bay  of  San  Francisco  is  at  San  Pablo  Bay.  The  Hay- 
wards  fauk,  which  skirts  the  western  flank  of  the  Berkeley 
Hills,  is  in  the  zone  of  dislocation  between  this  block  and  the 
block  on  whose  depressed  side  lies  the  greater  part  of  the  bay. 

Outside  of  the  Golden  Gate,  extending  out  to  the  Farallon 
Islands,  there  is  a  broad  submerged  embankment,  which  lies 
beneath  an  area  of  very  shallow  water.  (See  fig.  2,  p.  10.) 
This  embankment  probably  in  part  represents  the  delta  of  the 
ancient  river  that  once  flowed  through  the  Golden  Gate  before 
the  depression,  but  it  has  been  also  in  2:)art  built  up  by  deposits 
of  fine  silt,  which  in  the  flood  season  are  carried  through  the 
Bay  of  San  Francisco  and  dropped  outside  the  Gate. 


28 

GEOLOGY." 
STRATIGRAPHY  AND  AREAL  GEOLOGY. 

GEOLOGIC    FORMATIONS    OF    THE   MIDDLE    COAST    RANGES. 

The  middle  Coast  E,anges  of  California,  within  which  lie  the 
quadrangles  described  in  this  folio,  are  composed  of  many  dif- 
ferent kinds  of  rock,  both  igneous  and  sedimentary.  The  geo- 
logic history  of  the  region  is  varied,  including  records  of  depo- 
sition, erosion,  diastrophism,  and  volcanic  eruptions,  and  the 
geologic  structure  is  correspondingly  complex  and  interesting. 

The  formations  comprising  the  sedimentary  rocks  are  graph- 
ically represented  in  the  columnar  section  on  the  back  of  the 
Tamalpais  topographic  map  (in  pocket). 

The  oldest  known  rocks  are  certain  quartzites,  limestones, 
and  crystalline  schists,  which  are  best  exposed  in  the  Santa 
Cruz,  Santa  Lucia,  and  Gabilan  ranges.  The  age  of  these 
rocks  is  not  yet  known,  but  some  of  them  are  probably  early 
Mesozoic  and  some  are  possibly  Paleozoic. 

These  older  rocks  are  intruded  by  the  granitic  and  dioritic 
rocks  of  the  ranges  just  mentioned  and  their  extensions  north- 
ward through  Montara  Mountain,  the  Farallon  Islands,  and  the 
Point  Peyes  Peninsula  as  far  as  Bodega  Head. 

Upon  the  eroded  surface  of  the  complex  of  plutonic  and 
metamorphic  rocks  rests  the  Franciscan  group,  composed  chiefly 
of  sandstones,  radiolarian  chert,  foraminiferal  limestone,  and 
lavas,  associated  with  which  are  intrusive  masses  of  spheroidal 

"The  geologic  mapping  of  the  area  covered  by  this  foUo  has  afforded  an 
opportunity  for  training  in  field  geology  many  students  in  the  University  of 
California,  who  have  contributed  observations  that  are  recorded  in  the  text. 
The  list  of  these  contributors  is  long,  however,  and  the  same  ground  has 
been  worked  over  by  different  students,  so  that  it  is  impracticable  to  make 
individual  acknowledgments  for  the  aid  rendered.  The  nominal  author  of 
the  folio  is  familiar  with  all  parts  of  the  field  and  assumes  responsibility  for 
the  correctness  of  the  observations  made  and  conclusions  reached  in  the 
work  done  under  his  direction,  but  he  gratefully  acknowledges  his  ol)liga- 
tions  to  all  who  have  aided  in  this  work. 

In  the  study  of  the  fossiliferous  formations  the  author  has  had  the  active 
cooperation  of  Prof.  J.  C.  Merriam,  whose  contributions,  with  those  of  his 
students,  have  been  indispensable  in  unraveling  the  intricate  geology  of  the 
Concord  quadrangle  All  the  fossils  named  in  the  lists  here  published  have 
been  identified  by  Prof.  Merriam. 


29 

basalt  and  serpentinized  peridotite.  These  rocks  are  widely  dis- 
tri])iited  in  the  middle  Coast  Ranges,  oceuiTing  notably  in  the 
Mount  Hamilton  and  Mount  Diablo  ranges,  about  the  Bay  of 
San  Francisco,  and  in  areas  north  of  the  bay. 

Upon  the  Franciscan  group  the  Shasta  series  (Lower  Creta- 
ceous) rests  in  unconformable  relation,  and  upon  this  group  lies 
the  Chico  formation  (Upper  Cretaceous).  These  Cretaceous 
formations  were  once  coextensive  with  the  territory  now  occu- 
pied by  the  present  Coast  Ranges,  and  although  removed  by 
erosion  over  large  areas  where  the  Franciscan  and  older  rocks 
now  appear  at  the  surface,  they  still  constitute  one  of  the  largest 
elements  in  the  stratigraphy  of  the  region.  They  are  composed 
chiefly  of  shales  and  sandstones  and  in  the  ranges  north  of  the 
bay  have  a  measured  thickness  of  between  5  and  6  miles.  The 
Eocene  rocks,  which  succeed  the  Chico,  are  much  less  widely 
distributed.  They  comprise  two  assemblages  of  sandstones  and 
shales  known  as  the  Martinez  and  the  Tejon  formations,  which 
near  the  Bay  of  San  Francisco  aggregate  between  4000  and 
5000  feet  in  thickness  and  in  the  area  farther  south  are  prob- 
ably much  thicker.  Evidences  of  unconformity  between  the 
Eocene  and  Cretaceous  rocks  have  been  observed  in  some 
places,  but  the  discordance  is  not  very  pronounced.  The 
fossil  faunas  of  the  two  series  are,  however,  very  different. 

Some  strata  referable  to  the  Oligocene  series  have  been 
observed  and  recorded,  but  the  next  great  group  of  rocks  is  of 
Miocene  age  and  is  known  as  the  Monterey  group.  The  for- 
mations of  this  group  have  a  wider  distribution  south  of  the 
Bay  of  San  Francisco  than  those  of  Eocene  age,  and  in  some 
places  they  rest  directly  upon  Cretaceous  or  older  rocks,  neither 
the  Martinez  nor  the  Tejon  intervening.  The  most  character- 
istic feature  of  the  group  is  its  content  of  bituminous  shale, 
with  which  nearly  all  the  oil  of  California  is  directly  or  indi- 
rectly associated.  These  shales  alternate  with  sandstones,  and 
the  basal  formation  of  the  group  is  at  many  places  conglom- 
eratic. The  group  attains  a  thickness  of  several  thousand  feet, 
and  in  areas  where  it  rests  upon  the  Eocene  the  superposition 
is  unconformable.  The  rocks  of  this  group  were  doubtless 
originally  deposited  over  the  greater  part  of  the  area  of  the 


\0 


Coast  Ranges  from  the  Bay  of  San  Francisco  southward,  but 
since  their  deformation  and  uplift  they  have  been  extensively 
eroded.  Their  remnants,  however,  form  a  considerable  element 
of  the  stratigraphy  of  the  region. 

The  next  overlying  formation,  the  San  Pablo,  is  unconform- 
able with  the  Monterey  group  and  is  much  less  widely  distrib- 
uted. In  its  southern  areas  the  discordance  is  strongly  marked, 
but  in  some  of  the  northern  areas  it  is  scarcely  discernible. 
The  rocks,  which  are  chiefly  marine  sandstones  that  are  locally 
intermixed  with  tuffs,  are  found  on  both  flanks  of  the  Coast 
Ranges.  Their  thickness  in  the  best-known  sections  ranges 
from  1500  to  2000  feet. 

Above  the  San  Pablo  unconformably,  but  in  few  places 
resting  directly  upon  it,  lies  the  Merced  formation,  a  thick 
accumulation  of  marine  sandstones,  clays,  and  conglomerates, 
which  were  laid  down  in  Pliocene  time  in  deep  local  troughs 
that  sank  as  fost  as  the  sediments  were  deposited.  These 
basins  of  Pliocene  marine  deposits  were  apparently  confined 
to  the  coastal  side  of  the  Coast  Range  region.  On  the  inland 
side  of  that  region  similar  geosynclinal  troughs  were  devel- 
oped to  corresponding  depths,  in  which  accumulated  fluviatile 
and  lacustral  sediments,  constituting  the  Orinda"  formation. 
The  Orinda  and  Merced  formations  are  each  more  than  a  mile 
thick.  Interstratified  with  the  beds  of  both  formations  are 
layers  of  volcanic  ash. 

Upon  the  Orinda  and  Merced  lie  various  lavas  and  volcanic 
tuffs  alternating  with  lacustral  clays,  limestones,  and  sand- 
stones. Of  these  lacustral  formations,  the  Siesta"  (Pliocene) 
and  the  Campus"  (Pleistocene)  are  the  most  extensive. 

The  later  Quaternary  formations  comprise  marine  shell  beds, 
sands,  and  clays  overlain  by  a  thick  deposit  of  alluvium  that 
is  ricli  in  the  bones  of  extinct  Mammalia. 

"In  this  folio  the  names  Orindan,  Siestan,  and  Cauipan  have  been 
changed  to  Orinda,  Siesta,  and  Campus.  Tiie  names  orio:inally  eiuj^loyed 
have  been  in  use  for  more  than  13  years  antl  tlie  autlior  prefers  them. 


31 


PKE-FRANCISCAN    ROCKS. 
CHARACTER  AND  DISTRIBUTION. 


Tbe  oldest  rocks  of  that  part  of  tlie  Coast  Ranges  wliicli  is 
here  especially  considered  comprise  the  quartz  diorite  ("Mon- 
tara  granite")  and  some  fragments  of  the  formations  into 
which  that  rock  was  intruded  as  a  batholithic  mass.  In  the 
southern  Coast  Ranges  these  pregranitic  formations  constitute 
a  large  volume  of  strata,  of  unknown  age,  made  up  chiefly 
of  crystalline  limestones,  quartzites,  and  schists.  They  are 
exposed  on  the  flanks  of  the  Montara  batholith  in  Santa 
Cruz  County  (see  Santa  Cruz  folio,  No.  163)  and  on  the 
Point  Reyes  Peninsula  in  Marin  County,  according  to  F.  M. 
Anderson." 

GAVILAN   LIMESTONE. 

Only  one  of  the  pregranitic  formations,  the  Gavilan  lime- 
stone,'' which  takes  its  name  from  the  Gabilan"  Range,  between 
San  Benito  and  Salinas  valleys,  is  represented  in  the  area  here 
described.  It  occurs  only  in  isolated  masses  included  in  the 
quartz  diorite  of  Montara  Mountain,  in  the  southern  part  of 
the  San  Mateo  quadrangle.  This  limestone  is  a  coarsely  crys- 
talline marble  in  which  the  calcite  crystals  show  striations  due 
to  multiple  twinning.  Besides  the  carbonate  of  lime  of  which 
it  is  chiefly  composed  it  generally  contains  a  considerable  pro- 
portion of  magnesia,  some  silica,  and  some  carbon  in  the  form 
of  lustrous  flakes  of  graphite.  The  silica  may  occur  in  the 
form  of  silicates,  such  as  wollastonite,  which  is  abundant  in  the 
same  rock  on  the  Point  Reyes  Peninsula. 

QUARTZ  DIORITE   ("MONTARA  GRANITE"). 

The  so-called  "Montara  granite"  is  a  coarse-grained  gray  rock 
made  up  of  quartz  in  abundance,  plagioclase,  orthoclase,  and 
biotite  or  hornblende.  One  common  facies  of  the  mass  con- 
tains both  biotite  and  hornblende.     In  an  earlier  paper  the 

"California  Univ.  Dept.  Geology  Bull.,  vol.  2,  No.  5. 

*  Becker,  G.  F.,  U.  S.  Geol.  Survey  Mon.  13,  p.  181,  1888. 

"This  name,  Spanish  for  sparrow  hawk,  is  spelled  Gavilan  in  modern 
orthography  and  in  the  geologic  literature  of  California.  The  United  States 
Geographic  Board,  however,  has  decided  in  favor  of  the  older  form  Gabilan 
(pronounced  Gah-vee-lahn)  for  the  mountain  range. 


32 

writer  stated  that  the  rock  is  a  hornblende-biotite  granite,  but 
subsequent  examinations  have  shown  that  a  facies  of  the  mass 
which  contains  no  liorn blende  but  includes  biotite  is  perhaps 
more  widespread  than  that  in  which  hornblende  occurs  either 
in  association  with  biotite  or  alone.  It  was  found  also  that 
by  an  increase  in  the  proportion  of  plagioclase  the  rock  at 
many  places  passes  into  quartz  diorite,  so  that  this  designation 
is  petrographically  more  correct  for  the  mass  as  a  whole,  and 
the  term  granite  is  justified  only  by  popular  usage.  Titanite 
and  apatite  are  common  accessories  and  titanite  is  locally 
abundant.  The  rock  is  more  or  less  deeply  weathered  so  that 
it  is  disintegrated  in  almost  all  its  exposures  except  those  on 
the  shore  and  in  the  deeper  canyons,  where  corrasion  is  rapid. 
The  mass  is  at  some  places  characterized  by  blotchlike  inclu- 
sions of  a  dark,  more  basic  rock  and  contains  small  dikes  of 
aplite  and  pegmatite,  two  rocks  that  are  in  places  intimately 
associated  and  locally  include  small  crystals  of  garnet  and 
magnetite.  The  rock  shows  occasionally  a  foliated  or  gneissic 
structure,  which  is  clearly  due  to  deformation,  and  evidences  of 
deformation  appear  in  the  microscopic  structure  of  the  rock 
even  where  no  foliation  is  apparent. 

The  quartz  diorite  makes  up  the  bulk  of  Montara  Mountain, 
a  bold  ridge  nearly  2000  feet  high,  which  extends  from  north- 
west to  southeast  across  the  southwest  corner  of  the  San  Mateo 
quadrangle.  The  ridge  affords  two  complete  and  easily  acces- 
sible transverse  sections,  one  on  the  coast  bet^veen  San  Pedro 
Point  and  Halfmoon  Bay,  the  other  on  the  road  from  Crystal 
Springs  to  Spanish  Town,  along  Pilarcitos  Canyon,  just  beyond 
the  southern  border  of  the  quadrangle;  and  the  rock  is  well 
exposed  over  the  greater  part  of  the  surftice  of  the  ridge,  some 
of  the  slopes  being  mantled  with  a  coarse  arkose  sand  resulting 
from  its  disintegration. 

Along  the  northeastern  slope  of  Montara  Mountain  the 
quartz  diorite  is  flanked  by  sandstones,  grits,  shales,  and  con- 
glomerates of  probably  Eocene  age.  These  lie  directly  on  the 
worn  surface  of  the  quartz  diorite  and  dip  away  from  it  to 
the  northeast.  At  the  northwest  extremity  of  the  mountain 
the  sandstones   and  basal  conolomerate  mantle  over  the  axis 


of  the  ridge  and  rest  at  low  angles  upon  the  quartz  diorite,  as 
may  be  easily  seen  on  the  coast  road  near  the  Devils  Slide. 
At  Pilarcitos  Lake  the  plane  of  contact  of  the  sandstones  and 
quartz  diorite  has  a  lower  angle  than  the  slope  of  the  moun- 
tain, so  that  an  isolated  outcrop  of  the  underlying  rock  appears 
as  an  inlier  in  the  sandstone  at  the  south  end  of  the  lake.  At 
places  farther  north  these  Eocene  rocks  appear  to  be  faulted 
down  against  the  rocks  of  the  Franciscan  group  and  in  the 
earlier  reports  they  were  assigned  to  that  group,  although  their 
difference  from  the  normal  type  of  Franciscan  rocks  was  pointed 
out  and  the  desirability  of  segregating  them  was  suggested. 

In  the  area  southeast  of  Pilarcitos  Lake  the  quartz  diorite 
is  bounded  by  a  fault  that  brings  the  Franciscan  against  it. 
The  southern  limits  of  the  quartz  diorite  lie  beyond  the  San 
Mateo  quadrangle  and  are  mapped  in  the  Santa  Cruz  folio. 

On  the  strip  of  land  between  the  mountain  slopes  and  Half- 
moon  Bay  and  in  an  area  lying  farther  north,  between  that 
bay  and  Seal  Cove,  the  quartz  diorite  is  covered  by  alluvium 
composed  of  its  own  debris,  which  has  been  spread  out  by 
alluvial  fans  formed  by  streams  that  run  down  the  mountain 
slopes.  Near  Seal  Cove  the  quartz  diorite,  as  may  be  seen  in 
the  sea  cliffs,  forms  the  basement  on  which  rests  the  littoral 
Merced  formation.  Here  a  Pliocene  terrace  cut  in  the  quartz 
diorite  and  encumbered  by  beach  material  stands  only  a  few 
feet  above  the  present  sea  level.  From  Seal  Cove  to  the 
Devils  Slide  the  quartz  diorite  forms  the  shore  of  the  Pacific. 

The  quartz  diorite  area  of  Montara  Mountain  thus  outlined 
is  only  a  part  of  a  much  more  extensive  mass.  It  is  in  reality 
but  an  inlier  of  a  granitic  terrane  which  certainly  extends  from 
Santa  Cruz  to  Bodega  Head  and  which  emerges  from  beueath 
the  overlying  mantle  of  Mesozoic  and  Tertiary  sediments  in 
the  Santa  Cruz  Mountains,  Montara  Mountain,  the  Farallones 
Islands,  the  Point  Reyes  Peninsula,  and  Bodega  Head.  This 
great  batholith  has  an  extent  from  northwest  to  southeast  of 
not  less  than  120  miles  and  may  even  be  regarded  as  the 
continuation  of  the  similar  rocks  of  the  Gabilan  Range.  The 
mountain  mass  of  which  this  was  the  core  was  truncated  by 
erosion,  the  overlying  stratified  rocks  of  the  crust  into  which 


San  Francisco— 3 


34 


it  was  intruded  were  removed  except  some  remnants  on  its 
flanks,  and  a  large  part  of  the  quartz  diorite  itself  was  worn 
away  prior  to  the  submergence  which  permitted  the  deposition 
of  the  sediments  of  the  Franciscan  group. 

JURASSIC  (?)  ROCKS. 

FRAXCISCAX  GROUP  AND  ASSOCIATED  IGNEOUS  ROCKS. 

CHARACTER. 

The  Franciscan  group  was  named  from  San  Francisco,  where 
it  occurs  in  extensive  exposures,  from  which  it  was  first  described. 
It  comprises  (1)  a  vokimiuous  accumulation  of  sedimentary  for- 
mations, some  of  them  clearly  marine,  others  doubtfully  so; 
(2)  some  intercalated  lavas  of  contemporary  age;  and  (3)  cer- 
tain crystalline  schists  produced  by  the  metamorphism  of  both 
the  sedimentary  and  the  igneous  rocks. 

The  formations  of  the  Franciscan  group  are  pierced  at  many 
points  by  igneous  intrusives,  which  are  so  intimately  associated 
with  the  sedimentary  rocks,  both  as  to  age  and  as  to  distribu- 
tion, that  they  constitute  one  of  the  most  characteristic  features 
of  the  group  and  they  will  therefore  be  briefly  described  in 
connection  with  the  Franciscan  rocks.  This  treatment  is  also 
desirable  because  these  intrusives  produced  the  metamorphism 
that  formed  the  crystalline  schists  and  so  gave  to  the  Fran- 
ciscan group  one  of  its  most  interesting  features. 

The  sedimentary  rocks  of  the  group  comprise  (1)  sandstones, 
conglomerates,  and  shales;  (2)  limestone;  and  (3)  radiolarian 
cherts.  The  igneous  rocks  are  (1)  basalt  or  diabase,  in  many 
places  having  a  strongly  pronounced  spheroidal  or  ellipsoidal 
structure,  (2)  peridotites,  which  have  in  general  become  thor- 
oughly serpen tinized.  The  dominant  rock  in  the  cr\^stalline 
schists  is  glaucophane  schist,  which  is  so  abundant  in  them 
that  the  schists  as  a  whole  are  commonly  referred  to  as  "  the 
glaucophane  schists,"  although  other  varieties  of  crystalline 
schist  are  associated  with  them. 

STRATIGIIAPHY. 

Owing  to  the  general  absence  of  fossils  from  the  sandstones 
of  the  Franciscan  group  and  to  the  similarity  of  these  rocks  at 


S5 


several  horizons,  as  well  as  to  the  general  obscurity  of  their 
planes  of  stratification,  only  the  broader  features  of  the  stratig- 
raphy of  the  group  can  be  determined;  but  although  these 
sandstones  are  not  susceptible  of  subdivision  and  correlation 
by  means  of  their  petrographic  character  or  their  faunal  eon- 
tent,  two  radiolarian  chert  formations  that  occur  in  them 
constitute  well-defined  and  easily  recognizable  stratigraphic 
horizons  that  traverse  the  group.  As  neither  of  these  cherts 
forms  the  base  or  the  summit  of  the  group  they  serve  to  divide 
the  Franciscan  sandstones  into  three  distinct  formations,  which, 
with  the  cherts  themselves,  constitute  the  five  formations  of 
the  group.  Very  few  sections  of  the  Franciscan  rocks,  how- 
ever, include  all  five  of  these  formations.  Some  sections  com- 
prise only  the  lower  part  of  the  group,  others  only  the  upper 
part,  but  as  the  sequence  is  constant  these  partial  sections  may 
be  combined  to  construct  a  complete  stratigraphic  column. 

The  formations  which  by  this  mode  of  division  constitute 
the  Franciscan  group  are  as  follows,  the  series  beginning  at  tlie 
top: 

Bonita  sandstone. 

Ingleside  chert  (radiolarian  chert). 

Marin  sandstone. 

Sausalito  chert  (radiolarian  chert). 

Cahil  sandstone  (including  Calera  limestone  member  and 
some  volcanic  rocks). 

CAHIL,  MARIN,  AND   BONITA   SANDSTONES. 

Character. — The  Cahil  sandstone  is  named  from  Cahil  Ridge, 
in  the  San  Mateo  quadrangle;  the  Marin  from  Marin  Penin- 
sula, in  Marin  County;  and  the  Bonita  from  Bonita  Point,  on 
the  north  side  of  the  Golden  Gate.  These  three  formations 
are  petrographically  very  much  alike  and  may  therefore  be 
described  together.  The  prevailing  rock  in  all  three  formations 
is  a  massive,  obscurely  bedded  sandstone  of  dark  greenish-gray 
color  and  medium  texture.  Where  it  is  fresh,  or  unweathered, 
it  is  so  strongly  cemented  that  when  it  is  broken  the  fracture 
traverses  its  constituent  grains.  The  cementation  has  involved 
a  considerable  amount  of  recrystallization  of  the  finer  interstitial 


36 


material  of  the  rock,  but  the  larger  sand  grains  appear  to  have 
been  little  affected  by  this  secondary  crystallization  of  the 
matrix  in  which  they  are  embedded.  The  sand  of  which  it  is 
composed  was  not  well  washed  and  sorted  at  the  time  of  its 
deposition  and  comprises  several  other  minerals  besides  quartz, 
which  is,  of  course,  its  principal  constituent.  When  exam- 
ined under  the  microscope  it  is  seen  to  contain  more  abundant 
fragments  of  plagioclase,  orthoclase,  biotite,  hornblende,  and 
zircon  than  are  usually  found  in  rocks  of  this  class.  Besides 
these  minerals,  it  contains  pieces  of  chert,  volcanic  rock,  and 
schist.  This  heterogeneity  of  composition  and  the  prevailing 
angularity  of  the  grains  suggest  that  the  rock  may  be  an  arkose 
rather  than  a  beach-washed  sand.  It  includes  flakes  of  black 
carbonaceous  matter,  which  in  some  places  are  so  abundant  as 
to  o'ive  the  rock  a  rude  cleavage.  This  carbonaceous  material 
indicates  that  the  sands  w^ere  deposited  in  places  to  which  were 
carried  also  the  remains  of  vegetation.  In  certain  localities, 
too,  the  sandstone  includes  thin  seams  of  coal,  and  the  carbon- 
aceous fragments  are  exceptionally  abundant  near  these  seams. 
The  coal  is  of  no  commercial  value. 

The  sandstones  of  these  formations  include  lenses  of  pebbly 
conglomerate  and  beds  of  dark  shale,  but  most  of  these  are 
difficult  to  trace  for  more  than  short  distances  and  they  form 
beds  that  are  thin  in  comparison  with  the  general  mass  of  the 
sandstones. 

Where  erosion  is  not  exceptionally  active  the  sandstones  of 
these  formations,  by  their  secular  decay  and  disintegration,  yield 
an  abundant  soil.  Wherever  it  may  be  examined  in  cliff  sec- 
tions or  in  cuttings  the  sandstone  beneath  the  soil  presents 
a  more  or  less  shattered  appearance,  being  traversed  in  all 
directions  by  intei*secting  parting  planes.  Some  of  these  part- 
ings are  joints  or  planes  of  differential  movement,  but  most 
of  tliem  show  no  indications  w'hatever  of  movement  or  of  shat- 
tering action.  Nearly  all  of  these  partings  have  been  formed 
by  weathering,  and  their  development  seems  to  be  due  to  the 
slow  secular  disintegration  of  the  sandstone.  They  are  more 
numerous  near  the  surface,  and  they  have  divided  the  sand- 
stone immediately  beneath  the  soil  into  small  angular  pieces, 


37 


wliich,  after  they  are  loosened  by  surface  agencies,  become  incor- 
porated into  the  soil  itself.  The  sharply  marked  alternation 
of  wet  and  dry  seasons  and  the  general  absence  of  trees  in  this 
region  are  peculiarly  favorable  to  this  disintegration. 

In  steep  canyons  and  on  tlie  crests  of  ridges  these  massive 
sandstones  are  usually  well  exposed,  but  on  the  intervening- 
slopes  they  appear  only  in  isolated  knobs  that  protrude  above 
the  generally  smooth  surface,  so  that  the  distribution  of  the 
formations  must  be  to  a  large  extent  determined  by  examining 
the  rock  fragments  in  the  soil.  The  weathered  rock  is  com- 
monly of  a  tawny-yellow  color,  due  to  surface  oxidation,  and 
this  discoloration  ordinarily  extends  below  the  zone  of  mechan- 
ical disintegration  mentioned  above. 

Although  these  sandstones  are  prevailingly  massive,  signifi- 
cant glimpses  of  their  bedding  obtained  at  many  places  show 
that  they  are  normally  stratified.  The  bedding  in  these  places 
is  usually  made  apparent  by  intercalated  beds  of  shale  rather 
than  by  any  notable  differences  from  horizon  to  horizon  in  the 
character  of  the  sandstones  themselves.  The  sandstones  at 
certain  horizons  might  even  be  described  as  thin  bedded,  but 
where  this  thin  bedding  occurs  the  mantle  of  soil  is  heavy  and 
the  exposures  of  the  rock  are  few  and  small.  No  positive  indi- 
cation of  dynamic  metamorphism  has  been  observed  in  these  for- 
mations, but  where  they  lie  near  certain  intrusive  rocks  they 
have  been  aifected  by  a  peculiar  kind  of  contact  metamorphism, 
wdiich  has  produced  the  rocks  that  are  described  farther  along 
in  this  text  under  the  heading  "  Metamorphic  schists." 

In  the  Caliil  formation  there  is  a  conspicuous  foraminiferal 
limestone,  an  oceanic  deposit,  laid  down  far  from  the  shore, 
which  separates  the  sandstones  below"  and  above  it  into  distinct 
divisions.  The  attempt  to  indicate  this  separation  in  the  map- 
ping was,  however,  not  entirely  successful,  and  on  the  geologic 
maps  the  limestone  is  therefore  treated  as  a  member  of  the 
Cahil  sandstone,  under  the  name  Calera  limestone  member. 
On  the  summit  of  Fifield  Ilidge,  in  the  San  Mateo  quadrangle, 
a  thin  stratum  of  obscurely  fossiliferous  impure  limestone 
appears  as  a  lens  in  the  Cahil  sandstone  at  a  horizon  several 
hundred  feet  above  the  Calera  limestone.     It  has  been  observed 


88 


at  only  a  few  points  and  is  apparently  not  persistent.  This 
bed  probably  nowhere  exceeds  10  feet  in  thickness.  It  does 
not  resemble  the  Calera  limestone  -and  conld  not  easily  be  con- 
founded with  it. 

Owino-  to  the  obscurity  of  the  stratification  the  thickness  of 
the  Cahil,  Marin,  and  Bonita  sandstones  can  not  be  determined 
Avith  precision.  The  Cahil,  which  is  approximately  2560  feet 
thick,  is  made  up  of  about  60  feet  of  limestone  (Calera  lime- 
stone member)  with  500  feet  of  sandstone  below  and  2000  feet 
of  sandstone  above.  The  Marin  sandstone  in  its  best  expo- 
sure on  the  Marin  Peninsula  is  about  1000  feet  tliick  and  the 
Bonita  is  about  1400  feet  thick,  though  this  estimate  is  less 
certain,  as  tlie  exposures  are  obscure. 

These  three  formations  occur  principally  on  the  San  Fraur- 
cisco  and  Marin  peninsulas,  but  it  is  only  where  they  are 
immediately  associated  with  the  Sausalito  and  Ingleside  cherts 
that  they  can  be  distinguished  from  one  another  and  so  be 
assigned  to  their  proper  stratigraphic  positions  in  the  geologic 
sequence.  All  three  formations  are  remarkably  deficient  in 
fossils,  yielding  only  a  few  isolated  forms  or  fragments  of 
forms,  which  are  of  no  value  in  determining  the  age  of  the 
rocks. 

Calera  limestone  member  of  Cahil  sandstone. —  The  Calera 
limestone  is  so  named  from  Calera  Valley,  in  the  San  JNIateo 
quadrangle,  where  it  is  well  exposed  on  the  sea  cliffs  at  the 
lower  end  of  the  valley.  It  is  a  gray  compact  rock  of  aphan- 
itic  texture,  resembling  lithographic  limestone.  Its  weathered 
surface  is  much  lighter  in  color  than  its  freshly  broken  faces. 
Smooth  surfaces  or  thin  sections  of  the  rock  show  a  great  many 
clear,  hyaline  spots,  the  largest  measuring  half  a  millimeter 
across.  These  spots  are  the  remains  of  foraminiferal  shells. 
Under  the  microscope  the  limestone  appears  to  be  uniformly 
dense  and  structureless  and  is  composed  of  a  cryptocrystalline 
aggregate  of  calcite.  The  individuals  of  the  aggregate  are  so 
small  that  one  can  not  be  discriminated  from  another.  The 
character  of  the  limestone  and  the  distribution  of  the  forami- 
niferal shells  through  it  indicate  that  it  is  essentially  a  chem- 
ical precipitate    in  which   the  Foraminifera  were  sporadically 


39 


entombed.  Minute  veinlets  of  calcite  traverse  the  rock  in  all 
directions. 

The  limestone  is  more  or  less  distinctly  stratified  and  con- 
tains lenses  of  chert,  which  generally  lie  parallel  to  the  planes 
of  stratification.  (See  Pi.  III.)  These  lenses  are  rather  irreg- 
ular in  form  and  the  longer  axes  of  some  of  them  are  oblique 
to  the  bedding.  The  chert  consists  of  nearly  pure  silica  and 
may  be  either  dark  or  light  colored.  The  lenses  range  in 
thickness  from  about  an  inch  to  about  a  foot  and  have  ordi- 
narily rather  abrupt  or  obtusely  rounded  terminations.  Some 
of  them  appear  to  have  very  much  the  same  relation  to  the 
limestone  that  the  flint  nodules  and  lenses  in  the  chalk  of 
the  south  of  England  have  to  the  chalk. 

The  limestone  itself  contains  very  little  silica,  even  in  the 
immediate  vicinity  of  the  chert  lenses,  and  although  it  is  tra- 
versed by  numerous  small  veins  these  are  composed  of  calcite. 
The  chert  lenses  are  easily  separable  from  the  limestone,  which 
is  a  very  pure  carbonate  of  lime  with  but  little  admixture  of 
magnesium  carbonate,  as  may  be  seen  from  the  following 
analyses,  made  by  Wm.  L.  Lawsou,  of  samples  of  the  limestone 
from  Permanente  Canyon,  San  Mateo  County,  where  it  is 
quarried  for  use  in  the  manufacture  of  beet  sugar. 

Analyses  of  limestone  from  Permanente  Canyon,  San  Mateo  County,  Cal. 


1 

2 

CaO _     ._ 

54.44 

.20 

42.93 

.56 

.10 

1.98 

.05 

Trace. 

Trace. 

54.84 

MgO 

.14 

CO, 

AI2O3+ FOoOa  .___ 

43.23 

.47 

Soluble  SiOa 

Insoluble ......... 

HoO .     

1.52 

.15 

SO3 _. 

Trace. 

P,06 

Trace. 

100. 25 

100. 35 

40 

The  Calera  limestone  lies  in  a  belt  of  discontinuous  outcrops 
that  extend  from  northwest  to  southeast  across  the  southern 
part  of  the  San  Mateo  quadrangle.  The  thickness  of  the  lime- 
stone in  these  outcrops  varies  greatl}^  but  averages  about  60  feet. 

SAUSALITO   AND   INGLESIDE   CHERTS. 

General  character. — These  formations  consist  of  radiolarian 
chert  and  are  so  much  alike  that  one  description  will  serve  for 
both.  The  Sausalito  chert,  the  lower  of  the  two  formations, 
is  named  from  the  town  of  Sausalito,  in  the  Marin  Peninsula, 
near  which  it  is  extensively  exposed;  the  Ingleside  chert  tiikes 
its  name  from  Ingleside,  in  the  San  Francisco  Peninsula.  The 
rocks  of  w^hich  these  two  formations  are  composed  are  the  most 
remarkable  of  the  Franciscan  group.  They  are  neither  so  thick 
nor  so  persistent  as  the  sandstones,  but  their  great  hardness 
and  their  resistance  to  weathering  make  them  the  best-exposed 
formations  of  .the  group,  and  they  constitute  the  most  rugged 
features  of  the  relief.  The  interest  which  these  rocks  excite  by 
their  bold  outcrops  is  intensified  by  the  most  cursory  inspec- 
tion of  their  structural  and  petrographic  features.  In  color  they 
are  prevailingly  dull  brownish  red,  especially  in  their  thicker 
and  more  evenly  bedded  portions,  but  they  include  some  rock 
that  is  yellow  and  green.  White,  colorless,  blue,  purplish, 
gray,  brown,  and  black  cherts  are  also  seen,  but  these  colors 
are  usually  local  and  are  not  characteristic  of  extensive  masses 
of  the  rock.  Where  the  cherts  have  been  subjected  to  heat, 
as  at  their  contact  with  irruptive  rocks,  they  are  a  brilliant 
vermilion-red. 

One  of  the  most  remarkable  features  of  the  cherts  is  their 
bedding,  wdiich  is  well  displayed  in  numerous  excellent  sec- 
tions, not  only  ^  natural  outcrops  but  even  better  in  many 
rock  cuttings  that  have  been  made  in  the  hills  of  San  Francisco 
for  extending  and  grading  streets  and  for  procuring  road  metal. 
All  these  exposures  show  a  strikingly  constant  form  of  bedding 
the  essential  feature  of  which  is  the  alternation  of  thin  sheets 
of  chert  with  partings  of  shale.  (See  PI.  VIII.)  The  shale 
and  the  chert  are  usually  of  about  the  same  color.  The  thick- 
ness of  the  sheets  of  chert  in  the  typical  sections  generally 
ranges  from  about  1  to  3  or  4  inches,  averaging  perhaps  2  or  3 


41 

inches.  Some  beds  are  much  thicker,  but  the  sections  of  the 
chert  nevertheless  in  general  show  thin  and  even  bedding. 
The  shaly  partings  between  these  sheets  usually  range  from 
about  one-eighth  to  one-half  inch  in  thickness,  but  many  of 
them  are  mere  films.  As  the  formations  are  in  some  places 
exposed  in  sections  that  are  several  hundred  feet  thick  they 
present  the  remarkable  phenomenon  of  an  alternation  of  thou- 
sands of  layei-s  of  chert  with  as  many  layers  of  shale.  In  the 
common  red  phase  of  the  formations  the  regularity  of  this 
thin-sheeted  stratification  is  amazing.  In  other  phases,  in 
which  red  iron  oxide  is  not  so  abundant,  the  regularity  is 
nnich  less  marked  and  the  sheets  assume  lenticular  forms. 
In  these  less  ferruginous  phases  the  chert  beds  reach  their 
maximum  thickness  and  the  shaly  partings  their  minimum. 
In  these  phases  the  chert  beds  may  not  be  separated  by  shale 
and  may  be  several  feet  thick,  so  that  the  formations  may 
locally  present  a  massive  aspect  having  litttle  resemblance  to 
the  prevailing  thin-bedded  facies.  There  are  also  gradations 
from  the  massive  to  the  thin-bedded  variety. 

Petrographic  features. — The  radiolarian  cherts  are  not  petro- 
graphically  uniform.  In  many  places  they  are  true  jaspers 
and  have  been  so  designated  in  some  of  the  earlier  descriptions 
of  them.  In  other  places  the  silica  of  which  they  are  composed 
is  chiefly  amorphous  and  the  rocks  resemble  flint  or  hornstone. 
In  still  other  places  the  proportion  of  iron  oxide  and  other  pig- 
ment they  contain  is  so  large  that  they  are  not  cherty  but  are 
so  soft  that  they  can  be  easily  scratched  with  a  knife.  In  a  few 
phases,  notably  those  that  contain  no  coloring  matter,  they 
become  a  quartz  rock  that  is  not  flinty  or  jaspery  and  that  does 
not  differ  essentially  from  vein  quartz.  This  phase,  however, 
is  exceptional  and  is  probably  due  to  local  causes. 

If  a  selected  series  of  thin  sections  of  these  cherts  is  viewed 
under  the  microscope  they  present  a  gradation  from  those  that 
are  composed  almost  wholly  of  amorphous  or  isotropic  silica  to 
those  that  are  a  holocrystalline  aggregate  of  quartz  granules. 
The  most  isotropic  sections,  however,  exhibit  numerous  minute 
scattered  points  that  polarize  light  and  that  can  not  be  sharply 
separated,  even  by  the  highest  powers,  from  the  isotropic  base. 
These  points  are  not  inclusions;  they  are  centers  of  incipient 


42 


crystallization  in  the  amorphous  rock,  corresponding  to  the  prod- 
ucts of  devitrification  in  glass.  In  other  sections  these  centers  of 
crystallization  are  much  more  thickly  crowded  and  well-defined 
areas  composed  of  interlocking  granules  of  quartz  appear,  inter- 
locking also  with  the  isotropic  base.  The  actual  boundaries 
of  these  areas  can  be  made  out  only  with  difficulty  and  uncer- 
tainty, owing  to  the  fact  that  the  quartz  grains  are  under  molec- 
ular strain,  which  produces  undulatory  extinction  as  the  stage 
is  revolved  between  crossed  nicols.  In  still  other  sections  these 
areas  coalesce  and  the  proportion  of  amorphous  base  to  the 
whole  rock  is  very  small.  Finally,  some  sections  show  a  holo- 
crystalline  aggregate  of  interlocking  quartz  grains.  Most  of 
the  srains  are  under  molecular  strain,  as  is  shown  bv  undula- 
tory  extinction,  and  somewhat  resemble  chalcedony.  The  dis- 
crimination between  the  amorphous  and  the  crystalline  silica  is 
easy  in  those  varieties  of  the  rock  that  contain  little  iron-ore  pig- 
ment but  becomes  more  difficult  as  the  abundance  of  the  obscur- 
ing pigment  increases.  The  gradation  thus  observed  in  a  series 
of  thin  sections  prepared  from  specimens  taken  at  random 
seems  clearly  to  be  a  gradation  in  time  and  not  merely  a  grada- 
tion in  space.  It  indicates  different  stages  of  a  process  of  crys- 
tallization in  a  solid  amorphous  mass.  If  this  be  granted, 
there  seems  to  be  no  good  ground  for  doubting  that  in  general 
the  holoerystalline  cherts,  or  jaspers,  were  originally  amorphous 
silica,  and  that  they  have  reached  their  present  form  by  a  pro- 
cess of  crystallization  quite  analogous  to  that  of  devitrification 
in  volcanic  rocks. 

In  addition  to  these  general  petrographic  features  the  radio- 
larian  cherts  present  a  few  features  that  are  of  subordinate  or 
local  interest.  The  oxide  of  manganese  seems  by  paragenesis 
to  be  connected  with  the  cherts.  This  oxide  is  not  usually 
found  in  masses  but  appears  only  as  films  and  stains  in  the 
crevices  of  the  chert  and  along  its  bedding  planes.  In  some 
places  the  stain  is  so  abundant  that  the  body  of  the  rock  is 
locally  blackened.  In  the  chert  at  Red  Rock,  in  the  Bay  of 
San  Francisco,  a  tliick  deposit  of  oxide  in  the  form  of  psilome- 
lane  lies  in  the  bedding  planes,  which  ai-e  here  nearly  vertical. 
(Jther  similar  deposits  occur  in  these  cherts  in  the  Coast  Ranges. 


43 


So  far  as  the  writer  is  aware  the  manganese  ore  is  confined  to 
these  cherts.  Another  interesting  feature  of  these  rocks  is 
their  passage  locally  into  an  iron  ore.  Still  another  character- 
istic feature,  which,  however,  varies  in  the  degree  of  its  devel- 
opment, is  the  system  of  minute  fissures  that  traverse  the  hard 
beds  transverse  to  the  bedding  planes.  Many  of  these  fissures 
are  fault  planes  along  which  have  occurred  tiny  dislocations 
that  are  apparent  in  the  steps  which  mark  the  otherwise  even 
surfaces  that  form  the  upper  and  lower  limits  of  the  chert  beds. 
Some  of  these  fissures  are  lines  of  very  evident  veining,  and 
doubtless  all  of  them  Avould  prove  to  be  veins  if  examined  in 
thin  section.  The  vein  matter  is  quartz,  which  is  usually  white 
or  hyaline,  whereas  the  chert  is  colored.  None  of  these  fissures, 
faults,  or  veins  pass  tlirough  the  shaly  partings,  the  plasticity 
of  which  has  prevented  their  development,  so  that  each  thin 
sheet  of  chert  has  its  own  system  of  fissures. 

Fossils. — On  a  smooth  surface  of  almost  any  specimen  of 
these  cherts  a  lens  will  reveal  minute  round  or  oval  dark, 
hyaline,  or  wliitish  dots.  These  dots,  which  are  scattered 
through  the  rock,  prove  on  microscopic  examination  to  be  the 
remains  of  Radiolaria,  the  characteristic  fossils  of  these  forma- 
tions. The  Radiolaria  are  minute  animals  that  thrive  in  sea 
water  and  secrete  siliceous  skeletons  of  very  complex  structure. 
These  skeletons  evidently  accumulated  in  great  numbers  on 
the  floor  of  the  sea  while  the  radiolarian  cherts  were  being 
deposited  and  thus  contributed  to  their  formation.  As  a  rule 
they  are  sporadically  embedded  in  the  siliceous  matrix  above 
described,  but  in  some  places  they  are  so  closely  crowded  as  to 
constitute  the  greater  part  of  the  chert.  Where  the  Radiolaria 
are  scantily  distributed  through  the  chert  it  is  uncertain 
whether  or  not  the  matrix  also  is  derived  from  these  organ- 
isms, and  the  alternative  hypothesis  that  it  was  formed  by  the 
purely  chemical  precipitation  of  silica,  supplied  possibly  by 
submarine  springs,  is  wortliy  of  consideration.  If  the  silica  is 
wholly  of  organic  origin  it  must  have  been  dissolved  and 
repreci  pita  ted  in  its  present  form  as  an  ooze  on  the  sea  bottom. 

Under  the  microscope  the  radiolarian  remains  appear  in 
ordinary  light  as  circular  or  oval  spaces  or  as  clear  rings  free 


44 


from  pigment.  Between  crossed  nicols  these  clear  spaces  are 
seen  to  be  occupied  by  chalcedony.  The  clear  areas  are  more 
sharply  defined  in  the  amorphous  vaiieties  of  chert  and  they  are 
somewhat  indefinite  in  outline,  yet  distinct  as  areas,  in  the 
holocrystalline  varieties.  In  thin  section  they  are  most  readily 
observed  in  the  red  cherts,  by  reason  of  the  contrast  which 
they  make  with  the  pigmented  matrix. 

In  the  better  preserved  remains  of  these  organisms  the  spines, 
lattice  work,  and  other  structural  features  may  be  observed  and 
the  genera  to  which  they  belong  thus  determined.  Specimens 
of  radiolarian  cherts  from  the  Franciscan  group  and  their  fossils 
were  described  many  years  ago  by  Dr.  George  J.  Hinde." 

Distribution  and  thickness. — The  Sausalito  and  Ingleside 
cherts  are  most  extensively  displayed  in  the  hills  in  the  north- 
ern part  of  the  San  Francisco  Peninsula  and  in  the  southern 
part  of  the  Marin  Peninsula,  but  smaller  areas  of  these  cherts 
and  of  other  cherts  of  undefined  horizons  are  common  through- 
out the  Franciscan  terrane.  The  maximum  thickness  of  the 
Sausalito  chert  is  about  900  feet  and  that  of  the  Ingleside 
chert  about  530  feet.  The  absence  or  paucity  of  land-derived 
sediment  in  both  formations  indicates  that  they  were  deposited 
far  from  the  shore,  in  deep  water. 

CONDITIONS  OF  SEDIMENTATION. 

The  basal  part  of  the  Cahil  sandstone  was  deposited  upon 
the  sinking  bottom  of  a  transgressing  sea.  When  the  sub- 
sidence had  proceeded  so  far  that  the  land-derived  sediments 
failed  to  reach  the  deeper  water,  detrital  accumulation  gave  way 
to  the  formation  of  foraminiferal  ooze  on  the  bottom  of  a  clear- 
water  sea,  the  shore  having  migrated  far  to  the  east.  The 
product  of  this  period  of  deposition  is  the  Calera  limestone 
member.  An  upw^ard  movement  of  the  sea  bottom  caused  the 
shore  line  to  move  westward  again,  the  water  became  too  shal- 
low for  foraminiferal  life,  and  the  layer  of  ooze  on  the  sea 
bottom  was    buried  under  the  detrital  sediments  which  form 

"Hinde,  Gr.  J.,  Note  on  the  radiolarian  chert  from  Angel  Island  and 
from  Buriburi  Ridge:  California  Univ.  Dept.  Geology  Bull.,  vol.  1,  No.  7, 
pp.  285-240,  1894.  (Appendix  to  paper  by  F.  L.  Ransonie  on  the  geology  of 
Angel  Island.) 


45 

the  upper  part  of  the  Cahil  sandstone.  As  these  sediments  are 
very  thick  the  sea  bottom  must  have  continued  to  subside  dur- 
ing the  period  of  their  deposition.  As  subsidence  proceeded 
the  detrital  material  washed  from  the  receding  shore  again 
failed  to  reach  the  region,  and  organic  agencies  once  more 
resumed  sway.  This  time,  however,  calcareous  organisms  were 
replaced  by  those  which  secrete  silica  from  the  sea  water,  so 
that  the  sea  bottom  was  covered  with  radiolarian  ooze,  which 
eventually  consolidated  as  the  Sausalito  chert.  The  rhyth- 
mical oscillation  of  conditions  which  produced  the  remarkable 
alternation  of  layers  of  chert  and  shale  in  this  formation  has 
not  yet  been  explained  but  was  probably  due  to  alternating 
conditions  in  the  sea  water  which  affected  or  interrupted  the 
swarming  of  radiolarian  life.  The  accumulation  of  this  radio- 
larian ooze  was  stopped  by  a  recurrence  of  the  shallowing  of 
the  sea  and  the  return  of  the  shore  to  a  line  sufficiently  near  to 
insure  the  deposition  of  sands  upon  the  siliceous  deposits. 
These  sands  now  form  the  Marin  sandstone.  After  this  sand- 
stone had  accumulated  to  a  thickness  of  about  1000  feet  the 
subsidence  of  the  sea  bottom,  which  had  been  in  progress  dur- 
ing the  period  of  its  deposition,  caused  the  shore  again  to 
retreat  so  far  that  the  conditions  became  once  more  favorable 
to  marine  life,  and  another  deposit  of  radiolarian  ooze  was  laid 
down.  This  ooze  formed  the  Ingleside  chert.  Again  uplift 
set  in,  shallow  water  prevailed,  the  shore  was  close  at  hand, 
and  the  sands  of  the  Bonita  sandstone  were  deposited.  As 
these  sediments  attain  a  thickness  of  1400  feet  we  must  assume 
that  the  uplift  that  started  their  deposition  was  followed  by 
gradual  depression,  which  continued  during  their  accumulation. 

Thus  the  mere  consideration  of  the  character  of  the  forma- 
tions listed  in  the  fivefold  subdivision  of  the  Franciscan  group, 
taken  with  their  sequence  in  the  geologic  column,  leads  to  the 
recognition  of  a  remarkable  series  of  vertical  oscillations  of  the 
sea  bottom  and  of  a  consequent  series  of  horizontal  migrations 
of  the  sea  shore  during  the  time  occupied  by  the  deposition  of 
the  sediments  of  the  group. 

It  may  be  well  to  note,  however,  that  in  the  above  outline  of 
the  conditions  of  sedimentation  that  existed  during  the  deposi- 


46 


tion  of  the  Franciscan  formations  it  is  assumed  that  the  Cahil, 
Marin,  and  Bonita  sandstones  are  wholly  marine.  The  char- 
acter of  these  formations  suggests,  as  has  been  indicated  in  the 
description  of  them,  that  they  may  be  in  part  at  least  non- 
marine  or  continental  deposits.  If  this  should  prove  to  be  the 
fact  the  record  of  oscillation  and  the  consequent  migration  of 
the  continental  margin  to  and  fro  must  have  been  even  more 
complicated  than  if  they  were  marine. 

At  many  places  and  at  several  geologic  horizons  in  the  sand- 
stones of  the  Franciscan  group  there  are  small  isolated  areas  of 
radiolarian  chert  that  can  not  be  referred  to  either  the  Sausalito 
or  the  Ingleside  formation.  Such  areas  are  particularly  note- 
worthy on  Buriburi  Ridge,  in  the  San  Mateo  quadrangle. 
Most  of  these  isolated  patches  of  radiolarian  chert  are  residuals 
of  somewhat  larger  masses,  but  many  of  them  appear  to  have 
been  originally  discrete  lenses,  which  indicates  that  during  the 
deposition  of  the  sandstones  of  the  Franciscan  group  the  condi- 
tions were  from  time  to  time  locally  favorable  to  the  accumula- 
tion of  radiolarian  ooze. 

CONTEMPORANEOUS  VOLCANIC   ROCKS. 

The  volcanic  lavas  which  are  interstratified  with  the  sedi- 
mentary deposits  of  the  Franciscan  group  are  basaltic  rocks, 
which  are  generally  vesicular  or  amygdaloidal  and  which  in 
most  of  their  outcrops  are  greatly  decomposed.  Where  thus 
decomposed  they  are  as  a  rule  not  essentially  different  from 
other  basaltic  rocks  that  are  clearly  intrusive  in  the  Franciscan. 
The  masses  of  intrusive  rock,  however,  cut  boldly  and  iri'egu- 
larly  across  the  stratification  of  the  sedimentary  beds,  whereas 
tlie  volcanic  rocks  occur  in  sheets  of  moderate  thickness  that 
conform  to  the  planes  of  stratification  and  are  in  some  places 
associated  with  pyroclastic  rocks.  These  distinctly  interbedded 
lavas  are  much  more  common  in  the  lower  sandstone  forma- 
tions than  in  higher  portions  of  the  group.  The  most  note- 
worthy sheets  of  volcanic  rock  are  on  the  fianks  of  Cahil 
and  Sawyer  ridges,  in  the  San  Mateo  quadrangle.  They  are 
mapped  with  the  intrusive  basalt  and  diabase. 


IIS'TRUSIVE   ROCKS  ASSOCIATED   WITH   THE   FRAXCISCAN  GROUP. 

The  greater  part  of  the  igneous  rocks  associated  with  the 
Franciscan  group  are  not  of  contemporaneous  origin  with  it 
but  have  been  intruded  into  it  and  are  therefore  not  properly 
a  part  of  the  group.  Tliere  are  several  distinct  types  of  these 
intrusives,  two  of  which  are  dominant.  These  are  (1)  genetic- 
ally allied  peridotites/pyroxenites,  and  gabbros,  the  first  named 
preponderating  and  being  generally  very  thoroughly  serpen- 
tinized;  and  (2)  spheroidal  and  variolitic  basalts  and  diabases. 

Peridolite  and  serpentine. — The  intrusive  rocks  of  the  first 
class  may  be  referred  to  generally  as  peridotite  and  serpentine, 
for  although  tliey  are  associated  witli  pyroxenites  and  gabbros 
these  are  very  subordinate  in  amount  and  are  regarded  as  dif- 
ferentiation products  of  the  same  magma.  These  rocks  form 
dikes  and  laccolithic  sills  as  well  as  irregular  masses,  most  of 
which  appear  to  be  the  eroded  remnants  of  intrusive  lenses. 
Some  writers  on  Coast  Kange  geology  have  erroneously  sup- 
posed that  these  bodies  of  serpentine  are  products  of  the  meta- 
morphism  of  the  sandstone,  an  idea  that  still  finds  occasional 
expression,  although  it  has  been  entirely  disproved.  The  ser- 
pentine in  its  several  variations  has  the  usual  character  of  that 
rock.  Much  of  it  is  massive  and  unsheared,  but  in  places  it 
shows  abundant  evidence  of  pronounced  internal  movement, 
being  composed  of  slickensided,  sheared,  and  schistified  serpen- 
tine in  which  lie  rounded  masses  of  unsheared  rock.  The 
massive  varieties  contain  crystals  of  enstatite  that  show  lustrous 
cleavage  faces,  and  this  fact  and  the  results  of  microscopic  study 
indicate  that  the  serpentine  w'as  derived  from  a  peridotite  hav- 
ing the  mineralogic  character  of  saxonite  or  Iherzolite.  These 
massive  varieties  of  the  ser^^entine  are  also  usually  traversed  by 
numerous  small  veins  of  chrysotile,  but  none  of  these  veins  are 
large  enough  to  constitute  marketable  asbestos.  Small  seams 
of  magnesite  also  appear  here  and  there  in  tlie  serpentine. 
Bodies  of  glaucophane  and  other  crystalline  schists  lie  near 
some  of  the  masses  of  serpentine  but  are  too  small  for  repre- 
sentation on  the  geologic  maps. 

Perhaps  the  largest  areas  of  serpentinized  peridotite  lie  in 
the  belt  that  extends  across  the  city  of  San  Francisco  from 


48 


Hunter  Point  to  Fort  Point.  This  belt  apparently  includes 
two  distinct  sheets  of  serpentine,  which  are  intrusive  in  the 
Franciscan  group  and  are  separated  by  a  moderate  thickness  of 
Franciscan  rocks,  the  twofold  character  of  the  intrusion  being 
very  evident  in  the  excellent  exposures  at  Hunter  Point,  the 
Potrero,  and  Fort  Point.  These  sheets  lie  nearly  flat  but  are 
in  places  so  warped  that  they  are  more  oY  less  discordant  with 
the  stratification  planes  of  the  Franciscan  rocks.  The  intruded 
rocks  at  Fort  Point  are  seamed  with  veins  containing  magnesite, 
datolite,  pectolite,  and  other  minerals  but  apparently  have  not 
been  altered  by  contact  metamorphism  except  for  a  very  few 
inches  from  the  contact  surface.  Near  the  serpentine,  however, 
both  at  Fort  Point  and  at  Hunter  Point,  small  loose  frag- 
ments of  glaucophane  schist  have  been  found.  In  this  belt  of 
serpentine,  particularly  at  Hunter  Point,  there  are  stringers 
and  nests  of  chromite,  irregular  masses  of  chlorite  several  feet 
across,  and  small  veins  of  talc.  The  serpentine  is  nearly  every- 
where thoroughly  sheared  by  movements  that  have  been  set  up 
within  its  mass,  probably  in  connection  with  the  gradual  and 
irregular  increase  of  volume  attendant  upon  serpen tinization. 
This  sheared  mass,  which  is  traversed  in  all  directions  by  slick- 
ensided  surfaces,  contains  numerous  subangular  and  rounded 
bowlders  as  well  as  much  larger  masses  of  unsheared  serpentine, 
most  of  which  are  traversed  superficially  hj  a  roughly  rectang- 
ular network  of  minute  veins  of  chrysotile.  The  sheared  ser- 
pentine of  the  Potrero  and  of  Hunter  Point  incloses  many 
rather  short,  thick  lenses  of  fresh  fine-grained  diabase,  wdiich 
are  evidently  dislocated  fragments  of  dikes  that  were  intruded 
into  the  peridotite  before  its  serpentinization  and  which  by  this 
process  became  sheared  apart  into  discrete  lenses.  The  intru- 
sive character  of  these  diabase  lenses  is  shown  by  the  fact  that 
tlieir  edges  that  lie  against  the  serpentine  liave  been  chilled 
and  are  fine  grained  and  grade  into  a  coarser-textured  rock 
toward  the  centers  of  the  lenses. 

Another  belt  of  intrusive  peridotite  lies  along  Buriburi  and 
Pulgas  ridges,  on  tlie  northeast  side  of  Crystal  Springs  Lake, 
in  the  San  JNfateo  quadrangle,  where  it  is  also  thoroughly  ser- 
pentinized.     It  occurs  to  a  minor  extent  in  the  form  of  dikes 


49 


but  chiefly  in  the  form  of  lenticular  flat-lying  laccolithic  sheets, 
froin  which  the  overlying  formations  have  been  to  a  large 
extent  removed  by  erosion.  On  the  southwest  side  of  Fifield 
Kidge  there  is  a  zone  of  outcrops  of  sheets  and  dikes  of  ser- 
pentine which  is  smaller  than  that  on  Buriburi  Ridge  but  lies 
parallel  to  it. 

On  the  Marin  Peninsula  there  are  two  well-defined  belts  of 
serpentinized  peridotite.  One  of  these  extends  northwestward 
from  a  point  near  the  west  peak  of  Mount  Tamalpais  be3-ond 
the  northern  limit  of  the  Tamalpais  quadrangle.  The  serpen- 
tine here  occurs  in  the  form  of  a  rather  thin  warped  sheet, 
which  has  been  partly  exposed  by  the  removal  of  the  overlying 
formations,  into  which  it  is  intrusive,  and  has  been  so  dissected 
that  it  is  represented  by  several  separate  areas.  The  second 
belt  traverses  the  Tiburon  Peninsula  longitudinally  and  is  also 
in  the  form  of  intrusive  sheets  and  dikes,  with  glaucophane 
schists  at  the  contact  zones,  one  of  the  most  notable  of  which 
is  that  on  Angel  Island,  described  by  F.  L.  Ransome." 

Still  another  belt  of  serpentine  lies  on  the  east  side  of  the 
Bay  of  San  Francisco,  near  the  edge  of  the  Franciscan  group, 
wdiere  it  passes  beneath  the  overlying  Knoxville  formation. 
This  belt  extends  through  the  San  Francisco,  Concord,  and 
Haywards  quadrangles.  Glaucophane  schists  are  intimately 
associated  with  these  masses  of  serpentine,  particularly  at  the 
north  end  of  the  l)elr,  and  the  relations  of  the  two  rocks  indi- 
cate that  the  schists  are  due  to  metamorphism  at  the  contact 
with  the  peridotites.  Gabbros  and  pyroxenites  occur  as  local 
facies  of  the  serpentinized  peridotites.  Although  the  perido- 
tites from  which  the  serpentines  are  derived  are  clearly  mi  vu- 
sive  in  the  Franciscan  group  they  appear  to  be  conlin  o 
that  group;  they  do  not  traverse  the  overlying  Cretacious 
formation.  They  were,  however,  w^ell  exposed  by  denudation 
before  the  overlying  Knoxville  formation  was  deposited,  for 
they  formed  part  of  the  floor  on  wdiich  that  formation  was  laid 
down.  In  this  way  they  occur  in  some  places  at  the  contact  of 
the  Franciscan  with  the  overlying  shales,  and  as  both  shales 

"Ransome,  F.  L.,  The  geology  of  Angel  Island:  California  Univ.  Dept. 
Geology  Bull.,  vol.  1,  No.  7,  pp.  193-233,  1904. 

San  Francisco — 4 


50 


and  serpentine  have  been  deformed  the  relations  of  the  rocks 
are  in  places  extremely  involved,  suggesting  contacts  which 
really  do  not  exist. 

Silica-carbonate  rock. — Considerable  parts  of  the  serpentine 
have  been  altered  to  an  aggregate  of  carbonate  and  silica, 
which  weathers  in  very  rugged  masses  that  are  strongly  stained 
Avith  limonite.  This  rock  has  been  referred  to  in  an  earlier 
paper  as  silica-carbonate  sinter,  but  more  recent  studies"  have 
shown  that  the  rock  is  in  many  places  closely  associated  with, 
serpentine  and  in  some  places  grades  into  it,  so  that  there  is  little 
doubt  that  the  so-called  silica-carbonate  sinter  is  a  product  of  the 
alteration  of  the  serpentine,  the  bases  having  been  carbonatized 
and  the  silica  set  free  to  be  redeposited  in  the  form  of  chal- 
cedony, opal,  and  quartz.  These  forms  of  silica  occur  as  a 
plexus  of  small  veins  traversing  an  aggregate  of  carbonates  of 
iron,  magnesia,  and  lime.  Under  the  weather  the  carbonates 
are  dissolved  out  at  the  surface,  leaving  the  silica  prominent, 
as  well  as  rusty  with  iron  oxide.  The  more  notable  masses  of 
this  rock  are  found  north  of  Berkeley,  in  the  valley  of  Temescal 
Creek,  and  at  San  Bruno  Poiut,  but  many  other  smaller  bodies 
may  be  seen  elsewhere. 

Splieroidal  basalt  and  diabase. — The  second  class  of  intru- 
sives  invading  the  Franciscan  rocks  are  the  variolitic  and 
spheroidal  basalts  and  diabases.  These  are  confined  to  the 
Franciscan  group;  they  are  not  found  in  later  formations. 
They  antedate  the  peridotites,  which  at  many  places  appear  to 
cut  across  them.  These  masses  of  basalt  and  diabase  occur  at 
numerous  points  in  the  Franciscan  group  in  all  the  quadrangles 
and  thouo;h  most  of  them  are  small,  several  are  rather  larce. 
They  are  of  irregular  shape,  they  include  no  clean-cut  dikes 
or  intrusive  sills,  and  their  exposed  contacts  with  the  rocks 
they  intrude  are  generally  irregular  and  jagged.  (See  PI.  VI.) 
Fragments  of  the  incasing  rocks,  especially  of  the  radiolarian 
cherts,  are  abundant  at  the  contacts,  where  the  chert  is  usually 
baked  to  a  bright  vermilion-red  and  its  structure  is  in  some 
places  also  greatly  changed.     Some  inclusions  show  evidence  of 

"Knopf,  Adolph,  An  alteration  of  Coast  Range  serpentine:  California 
Univ.  Dept.  Geology  Bull.,  vol.  4,  No.  18,  1906. 


51 

partial  resorption.  The  spheroidal  structure  of  these  intrusive 
rocks  is  clearly  revealed  only  on  sea  cliifs,  as  at  Hunter  Point 
and  Point  Bonita  (see  PL  VII),  but  may  also  be  detected  in 
numerous  road  cuttings  and  on  natural  exposures  on  hillsides. 
On  sea-cliff  exposures  the  rock  presents  the  appearance  of 
an  irregular  pile  of  filled  sacks,  each  sack  having  its  rotundity 
deformed  by  contact  with  its  neighbor.  The  average  dimen- 
sions of  these  sacklike  or  ellipsoidal  masses  are  about  3  feet  in 
the  longest  and  about  1  foot  in  the  shortest  diameter.  The  rock 
between  the  ellipsoids  is  usually  more  decomposed  than  that 
elsewhere  and  so  weathers  out  easily  under  the  action  of  the 
waves,  leaving  the  more  resistant  ellipsoids  prominent.  Some 
of  the  ellipsoids  are  vesicular,  others  are  variolitic,  and  still 
others  are  both.  The  cause  of  this  peculiar  structure  and  the 
mode  of  its  development  are  not  yet  understood.  At  some 
places,  as  on  the  cliffs  at  Hunter  Point,  these  spheroidal 
basalts  present  also  a  remarkable  brecciated  appearance,  as  if 
made  up  of  innumerable  small,  angular  fragments  cemented 
together.  At  both  Point  Bonita**  and  Hunter  Point  the  mar- 
gins of  such  spheroidal  masses  show  a  pronounced  variolitic  struc- 
ture. At  Hunter  Point  the  rock  is  in  places  filled  with  closely 
packed  spherical  varioles  ranging  in  size  from  one-sixteenth  to 
one-half  inch.  These  varioles  weather  out  of  the  rock  easily, 
being  much  harder  than  the  matrix  in  which  they  are  embedded. 

The  intrusive  rocks  of  Angel  Island  described  by  Ransome'' 
under  the  name  fourchite  are  probably  a  fjicies  of  the  rocks 
here  grouped  under  the  general  designation  "Spheroidal  basalt 
and  diabase." 

These  spheroidal  basalts  occur  wherever  the  rocks  of  the 
Franciscan  group  are  exposed  in  this  area,  but  most  abundantly 
on  the  JMarin  Peninsula,  the  largest  masses  being  in  the  canyon 
of  Lagunitas  Creek  northwest  of  Mount  Tamalpais. 

"Ransome,  F.  L.,  The  eruptive  rocks  of  Point  Bonita:  California  Univ. 
Dept.  Geology  Bull.,  vol.  1,  No.  3,  1893. 

''Ransome,  F.  L.,  The  geology  of  Angel  Island;  California  Univ.  Dept. 
Geology  Bull.,  vol.  1,  No.  7,  1S94. 


52 


METAMORPHIC   ROCKS. 


One  of  the  most  interesting  features  of  the  Franciscan  group 
consists  of  crystalline  schists  formed  by  the  local  metamor- 
phism  of  its  sedimentary  and  igneous  rocks.  These  schists 
vary  considerably  in  petrographic  character  from  place  to  place 
and  are  of  widespread  though  sporadic  occurrence.  Most  of 
the  areas  they  occupy  are  small,  many  of  them  comprising 
only  an  acre  or  two,  but  some  of  them  include  hundreds  of 
acres.  These  schists  are  more  or  less  intimately  associated  with 
the  serpentinized  peridotites  or  at  some  localities  with  the 
spheroidal  basalts  and  diabases.  They  are  usually  exposed  in 
discrete  areas  on  the  periphery  of  the  intrusive  masses  but 
occasionally  appear  to  be  inclusions.  In  several  places  where 
these  metamorphic  rocks  are  dissociated  from  intrusive  bodies 
the  dissociation  is  probably  due  to  erosion.  The  serpentinized 
peridotites  were  in  many  places  originally  intruded  as  lacco- 
lithic  lenses,  which  have  been  removed  by  denudation,  and  the 
more  resistant  crystalline  schist  wdiich  lay  on  their  peripheries 
or  beneath  them  form  knobs  that  project  from  the  general 
surface.  Exceptionally  the  metamorphic  rock  forms  a  contin- 
uous belt  along  the  margin  of  an  intrusive  mass.  The  process 
of  alteration  seems  to  have  been  erratic  or  selective  in  its  oper- 
ation, so  that  the  metamorphic  rocks  occur  in  patches,  various 
in  form  and  size,  which  are  disposed  in  a  general  way  about 
the  margins  of  the  intrusive  masses.  In  this  respect  these 
metamorphic  rocks  are  quite  different  from  those  wdncli  form 
a  contact  zone  due  to  pure  thermal  metamorphism  on  the 
periphery  of  a  granitic  intrusion,  a  zone  that  is  commonly 
persistent  and  uniform.  The  metamorphism  in  the  Francis- 
can group  apparently  occurred  only  at  certain  localities  on  the 
peripheries  of  the  intruding  bodies,  either  where  the  emana- 
tions from  the  intrusive  mass  found  freer  escape,  or  where 
the  incasing  rocks,  by  reason  of  their  composition,  were  more 
susceptible  to  alteration,  or  wdiere  there  was  a  combination  of 
these  two  favorable  conditions.  The  composition  of  a  part  of 
the  metamorphic  rocks  suggests  that  this  part  was  derived  from 


53 


basic  igneous  rocks  preexistent  in  the  Franciscan,  but  other 
parts  are  evidently  altered  sedimentary  rocks.'^ 

Considered  petrographically  these  metamorphic  rocks  may 
be  grouped  in  several  classes,  among  which  are  (1)  amphibo- 
litic  schists,  (2)  mica  schists,  (3)  quartz  schists,  and  (4)  albite 
schists. 

The  amphibolitic  varieties  include — 

1.  Black  or  dark-green  schists,  composed  chiefly  of  horn- 
blende, which  is  usually  accompanied  by  some  albite  or  quartz. 

2.  Hornblende-garnet  schists. 

3.  Actinolite  schists,  with  or  without  chlorite  and  talc.  One 
facies  of  these  actinolite  schists  occurs  in  the  form  of  nodules 
in  serpentine,  the  prisms  of  actinolite  being  long  and  thick. 

4.  Glaucophane  schists,  composed  almost  wholly  of  glauco- 
pliane. 

5.  Glaucophane-quartz  schists. 

6.  Glaucophane-albite  schists. 

7.  Glaucophane-epidote  schists. 

8.  Glaucophane-lawsonite  schists. 

9.  Crocidolite-quartz  schists. 

10.  Massive  aggregates  of  green  or  blue  amphibole,  ompha- 
cite,  and  the  garnet  known  as  eclogite. 

The  micaceous  varieties  comprise  (1)  biotite-quartz  schists, 
(2)  muscovite-glaucophane  schists,  and  (3)  schists  in  wdiich 
muscovite,  glaucophane,  garnet,  and  quartz  are  about  equally 
developed. 

The  quartz  schists  comprise  varieties  in  which  the  dominant 
constituent  is  quartz,  with  wdiicli  are  associated  subordinate 
amounts  of  glaucophane  or  mica  or  crocidolite.  In  the  albite 
schists  the  dominant  constituent  is  feldspar,  which  is  accompa- 
nied by  subordinate  amounts  of  glaucophane  or  crossite.  All 
these  schists  of  course  contain  numerous  accessory  minerals, 
and  some  of  them  include  large  amounts  of  titanite. 

"Tliese  inetauiorphic  rocks  are  described  more  fully  in  several  papers, 
particularly  in  The  geology  of  Angel  Island,  by  F.  L  Ransome  (California 
Univ.  Dept.  Geology  Bull.,  vol.  1,  No.  7,  1894),  and  in  The  paragenesis  of  the 
minerals  in  the  glaucophane-bearing  rocks  of  California,  by  J.  P.  Smith 
(Am.  Piiilos.  Soc.  Proc.  vol.  45,  1907).  Other  papers,  dealing  chiefly  with 
the  mineralogy  of  the  rocks,  are  mentioned  in  the  bibliographic  list  at  the 
end  of  this  folio. 


54 


The  mere  mineralogic  description  and  nomenclature  of  these 
interesting  rocks,  however,  form  only  the  first  step  toward 
solving  the  intricate  problem  of  their  genesis.  The  fact  that 
they  are  derived  from  various  kinds  of  rocks,  both  sedimentary 
and  igneous,  and  yet  have  common  features,  such  as  their  con- 
tent of  blue  amphibole,  suggests  that  some  regional  condition 
influenced  their  development.  The  fact  that  some  of  the  most 
characteristic  minerals  of  the  body  of  the  schists — for  example, 
lawsouite — also  occur  in  veins  that  traverse  the  same  schists 
indicates  that  the  agencies  of  metamorphism  must  have  been 
somewhat  like  those  that  fill  veins,  and  that  the  metamorphism 
is  therefore  not  purely  thermal.  The  further  fact  that  a  char- 
acteristic mineral  of  these  schists,  blue  amphibole,  is  developed 
in  different  kinds  of  rocks  without  deformation  indicates  that 
the  agencies  that  formed  the  schists  were  not  the  forces  of 
dynamic  metamorphism.  If  purely  thermal  contact  metamor- 
phism and  dynamic  metamorphism  be  thus  both  set  aside  as 
possible  agencies  in  producing  the  schists  the  only  remaining 
explanation  of  their  origin  is  that  they  are  due  to  metamor- 
phism by  chemical  reactions,  which,  however,  have  not  greatly 
modified  the  original  chemical  composition  of  the  rocks  affected. 
The  nature  of  these  reactions  is  at  present  obscure,  but  they 
are  doubtless  various  and  complicated,  and  their  localization  is 
no  doubt  intimately  connected  with  emanations  from  intrusions 
of  basic  magma. 

These  inetamorphic  rocks  occur  most  extensively  in  the  San 
Francisco  quadrangle,  at  places  north  of  Berkeley,  and  on  the 
Tiburon  Peninsula. 

AGE  OF  THE  FRANCISCAN  GROUP. 

The  age  of  the  Franciscan  group  has  not  been  positively 
determined  either  in  the  quadrangles  here  mapped  or  elsewhere 
in  the  Coast  Ranges.  Neither  the  Foraminifera  of  the  Calera 
limestone  nor  the  Ivadiolaria  of  the  Sausalito  and  Tngleside 
cherts  appear  to  be  sufiiciently  distinctive  to  determine  the  age 
of  the  rocks  in  which  they  are  found.  Certain  plant  remains 
that  were  discovered  by  Fairbanks  in  the  Franciscan  rocks  at 
vSlate  Springs,  Monterey  County,  are  regarded  by  some  paleo- 


55 

botanists  as  forms  that  represent  an  age  transitional  from 
Jurassic  to  Cretaceous  time — Cretaceous  rather  than  earlier. 
F.  H.  Knowlton,  however,  expresses  the  opinion  that  these 
fossil  plants  may  be  either  Cretaceous  or  Jurassic.  Two 
marine  Mollusca,  one  a  single  specimen  of  an  Inoceramus, 
found  many  years  ago  on  Alcatraz  Island,  in  San  Francisco 
Bay,  the  other  a  fragment  of  either  an  Inoceramus  or  an 
Aucella,  found  2f  miles  south  of  San  Mateo,  afford  the  only 
other  paleontologic  evidence  available.  These  forms  indicate 
a  Jurassic  or  Cretaceous  age  but  are  otherwise  indecisive. 

Although  the  paleontologic  evidence  as  to  the  age  of  the 
Franciscan  group  is  thus  extremely  meager  and  inconclusive 
the  stratigraphic  evidence  is  curiously  self-contradictory  and 
leads  to  a  conclusion  that  few  geologists  may  be  willing  to 
accept.  The  rocks  of  the  Franciscan  group  are  overlain  uncon- 
formably  by  the  Knoxville  formation,  wdiich  is  generally 
regarded  by  paleontologists  as  Lower  Cretaceous.  Exposures 
of  the  Knoxville  resting  upon  the  eroded  surface  of  the 
deformed  and  locally  metamorphosed  rocks  of  the  Franciscan 
group  occur  on  the  southwest  flank  of  the  Berkeley  Hills, 
particularly  north  of  Berkeley,  and  similar  exposures  occur  in 
the  San  Luis  quadrangle  and  have  been  described  by  Fair- 
banks. The  relations  exhibited  in  these  exposures  would 
indicate  that  the  Franciscan  is  pre-Cretaceous  in  age.  Unfor- 
tunately for  the  stability  of  this  conclusion,  however,  other 
available  evidence  leads  to  another  decision.  In  several  folios 
of  the  United  States  Geological  Survey  the  granite  of  the  Sierra 
Nevada  is  mapped  and  described  as  of  post-Jurassic  age.  This 
granitic  rock  extends  continuously  from  the  southern  Sierra 
Nevada,  in  the  Mount  Whitney  region,  around  the  southern 
end  of  the  Great  Valley  of  California,  into  the  Coast  Ranges, 
where  it  forms  a  part  of  the  complex  upon  which  the  Fran- 
ciscan rocks  rest  unconformably,  as  has  been  indicated  in  the 
San  Luis  and  Santa  Cruz  folios.  In  the  area  here  mapped 
the  Franciscan  rocks  are  nowhere  superposed  upon  the  granitic 
rock,  which  is  confined  to  Montara  Mountain  and  has  the  facies 
of  a  quartz  diorite.  In  an  earlier  paper  the  writer  described 
the  Franciscan  as  resting  u|)on  this  quartz  diorite  ("Montara 


56 


granite"),  but  later  field  work  has  shown  this  to  be  an  error, 
the  supposed  Franciscan  being  in  reality  a  block  of  Eocene 
strata  faulted  against  the  Franciscan,  Where  the  Franciscan 
lies  in  contact  w^ith  the  quartz  diorite  in  the  San  Mateo  quad- 
rangle the  juxtaposition  is  due  to  faulting.  Nearly  all  writers 
on  Coast  Range  geology  are,  how^ever,  agreed  that  the  Fran- 
ciscan is  later  than  the  granitic  rocks.  The  considerations 
that  support  this  view  may  be  summarily  stated  as  follows : 

1.  The  sandstones  of  the  Franciscan  group  are  composed 
very  largely  of  granitic  debris,  as  was  first  pointed  out  by 
Becker,  who  regarded  these  rocks  as  Lower  Cretaceous. 

2.  At  no  place  has  the  granite  been  observed  to  be  intrusive 
in  the  Franciscan. 

3.  The  intrusive  relation  of  the  granite  to  the  older  rocks  is 
clearly  shown  at  many  localities. 

4.  Included  fragments  of  these  older  rocks  are  very  common 
in  the  granite,  but  no  fragments  of  Franciscan  strata  have  been 
found  in  the  granite,  although  the  radiolarian  cherts  are  well 
adapted  to  preservation  as  inclusions,  as  is  shown  b}^  the  fact 
that  they  are  common  in  the  rocks  that  intrude  the  Franciscan 
strata. 

5.  The  pregranitic  rocks  are  generally  metamorphosed  and 
consist  chiefly  of  coarsely  crystalline  marbles  (which  are  locally 
graphitic),  quartzites,  and  various  crystalline  schists,  none  of 
wdiich  resemble  the  Franciscan  strata. 

6.  The  local  metamorphism  of  the  Franciscan  is  not  due  to 
granitic  intrusion  but  is  intimately  associated  with  basic 
irruptives. 

7.  The  Franciscan  rocks  have  not  been  subjected  to  the 
same  dynamic  metamorphism  that  lias  generally  affected  the 
rocks  into  which  the  granite  is  intruded. 

8.  In  general,  there  are  in  the  Coast  Ranges  two  different 
assemblages  of  very  diverse  sedimentary  rocks,  into  one  of 
which  the  granite  is  clearly  intrusive,  whereas  the  other  con- 
tains no  such  intrusions;  therefore  it  is  highly  probable  that 
the  latter  assemblage  is  postgranitic. 

Considering,  then,  the  fact  that  the  Franciscan  is  post- 
granitic,  and  accepting  the  view  that  the  granitic  rocks  of  the 


57 


Coast  Ranges  are  continuous  with  and  of  the  same  age  as  the 
granite  of  the  Sierra  Nevada,  we  must  conclude  that  the  Fran- 
ciscan group  is  post-Jurassic.  This  conchision  is  clearly  in 
conflict  with  that  drawn  from  the  fact  that  the  Franciscan  lies 
unconformably  below  the  Knoxville  (Lower  Cretaceous).  At 
present  there  appears  to  be  no  way  of  harmonizing  the  conflict 
without  (1)  either  extending  the  geologic  time  at  the  interval 
between  the  recognized  Cretaceous  and  the  Jurassic,  an  exten- 
sion that  should  not  be  made  without  the  justification  of  more 
thorough  investigation,  or  (2)  assuming  a  period  of  batholithic 
development  in  the  Coast  Ranges  that  was  distinct  from  and 
older  than  that  in  the  Sierra  Nevada,  an  assumption  that 
should  also  not  be  made  without  further  and  fuller  investigation. 
It  is  hoped  that  this  statement  of  the  difficulty  of  determin- 
ing the  age  of  the  Franciscan  group  may  stimulate  California 
geologists  to  make  further  field  studies  directed  to  the  solution 
of  the  problem. 

CRETACEOUS    SYSTEM. 
DISTRIBUTION. 

The  Cretaceous  formations  constitute  an  important  feature 
of  the  geology  of  tlie  region  mapped  but  are  confined  to  the  east 
side  of  the  Bay  of  San  Francisco.  Their  chief  outcrops  lie  in 
a  belt  that  extends  through  the  San  Francisco,  Concord,  and 
Haywards  quadrangles,  widening  toward  the  southeast.  An 
area  of  the  upper  part  of  the  system  occurs  also  in  the  northern 
part  of  the  Concord  quadrangle. 

The  Cretaceous  rocks  lie  unconformably  upon  those  of  the 
Franciscan  group  along  the  southwest  front  of  the  Berkeley 
Hills  and  are  the  dominant  rocks  of  that  range.  They  strike 
northwest  and  dip  generally  to  the  northeast  at  rather  high 
angles,  although  in  places  the  dip  is  reversed  by  irregular 
plication  of  the  strata.  On  the  northeast  side  of  the  range 
the  upper  formation  of  the  Cretaceous  rocks  is  unconform- 
ably overlain  by  Tertiary  strata  that  occupy  a  great  trough  in 
the  central  part  of  the  Concord  quadrangle.  The  outcrop  of 
the  Cretaceous  rocks  along  the  Berkeley  Hills  is  thus  the 
southwest  limb  of  a  great  synclinorium,  tlie  northeast  limb  of 


58 

which  appears  in  the  northern  part  of  the  Concord  quadrangle, 
south  of  the  town  of  Martinez,  and  also,  more  extensively,  in 
the  Mount  Diablo  quadrangle,  which  adjoins  the  Concord 
quadrangle  on  the  east.  Of  these  two  areas  of  Cretaceous 
rocks,  that  of  the  Berkeley  Hills  is  stratigraphically  the  more 
important,  for  it  affords  complete  sections  of  the  system  from 
the  Franciscan  to  the  Tertiary,  and  it  will  therefore  be  de- 
scribed in  greater  detail. 

The  system  as  represented  in  these  quadrangles  comprises  two 
distinct  formations,  which,  named  in  ascending  sequence,  are 
designated  the  Knoxville  formation  and  the  Chico  formation. 

LOUVER  CRETACEOL^S  (SHASTA)  SERIES. 

KNOXVILLE  FORMATION". 

The  Knoxville  formation,  named  for  Knoxville,  in  Napa 
County,  is  exposed  in  a  practically  continuous  belt  along  the 
southwest  slope  of  the  Berkeley  Hills,  being  confined  to  the 
lower  part  of  the  slope  in  the  northwestern  part  of  the  range 
but  reaching  the  summit  in  the  'southeastern  part  and  de- 
scending to  the  foot  of  the  slope  beyond  Haywards.  The 
belt  widens  notably  from  northwest  to  southeast,  across  the 
Concord  quadrangle,  its  width  at  Berkeley  being  onl}'"  a  few 
hundred  feet,  whereas  at  the  southern  border  of  the  quadrangle 
it  is  over  tliree-quartei*s  of  a  mile.  Farther  southeast,  in  the 
Haywards  quadrangle,  the  belt  of  Knoxville  rocks  may  be 
still  wider,  but  the  rocks  are  in  part  mantled  over  by  later 
rhyolite  or  by  the  alluvium  of  the  valley  floor,  and  their  extent 
beneath  these  deposits  can  not  be  positively  stated.  The  widen- 
ing of  the  belt  of  outcrop  toward  the  southeast  signifies  a  thick- 
ening of  the  formation,  although  it  is  in  part  due  to  plication 
of  the  strata,  to  which  the  increased  volume  of  overlying  soft 
rocks  is  conducive.  The  maximum  thickness  of  the  forma- 
tion in  the  Concord  quadrangle  is  not  less  than  1000  feet  and 
may  ])e  as  much  as  1500  feet.  Tlie  formation  thins  out  rapidly 
northwest  of  Berkeley  and  at  its  northwestern  extremity  is 
represented  by  outlying  patches  that  rest  upon  the  worn  surface 
of  Franciscan  rocks.  Tliis  thinning,  however,  is  probably  due 
in  large  part  to  erosion  in  Tertiary  time,  for  these  remnants 


59 


are  uucoiiformably  overlain  by  Pliocene  and  Quaternary  for- 
mations. 

The  Knoxville,  being  the  basal  formation  of  a  series  that 
rests  unconformably  upon  tlie  Franciscan  rocks,  might  reason- 
ably be  expected  to  consist  of  tlie  coarse  detritus  that  is  charac- 
teristic of  transgressive  beds — that  is,  it  should  be  in  large 
measure  conglomeratic.  This,  however,  it  is  not,  and  the  pre- 
vailing absence  of  conglomerate  in  a  formation  so  situated  is 
interesting  and  significant.  The  formation  is  composed  almost 
wholly  of  dark,  more  or  less  carbonaceous,  argillaceous,  or  finely 
arenaceous  shale  but  includes  occasional  small  lenses  of  com- 
pact gray  limestone.  In  some  sections  it  consists  of  alternate 
beds  of  shale  and  thin  beds  of  rather  fine  sandstone;  in  others, 
particularly  in  the  San  Francisco  quadrangle,  it  contains  beds 
of  fine  pebbly  conglomerate,  the  pebbles  being  on  the  average 
not  much  larger  than  peas.  Freshly  exposed  sections  show 
that  this  shale  is  evenly  and  thinly  laminated,  but  owing  to  the 
readiness  with  which  it  slacks  under  the  atmosphere  and  breaks 
down  into  soil  fresh  exposures  are  uncommon,  and  the  dip  of 
the  strata  can  not  be  easily  observed  even  in  road  cuttings  only 
a  few  years  old.  The  stratification  in  the  more  recent  expo- 
sures, however,  indicates  that  the  formation  as  a  whole  dips  to 
the  northeast,  beneath  the  next  higher  formation.  Locally  the 
dip  is  reversed,  as,  for  example,  north  of  Laundry  Farm,  in  the 
southwestern  part  of  the  Concord  qujidrangle,  wdiere  the  shale 
wraps  around  an  area  of  underlying  serpentine  in  anticlinal 
fashion,  and  also  near  the  southern  border  of  the  same  quad- 
rangle, where  a  synclinal  fold  occurs. 

The  shaly  character  of  the  Knoxville  formation  is  not  merely 
a  local  anomaly  but  is  a  general  feature,  which  prevails  over  a 
large  part  of  the  Coast  Ranges  of  California,  This  fact  would 
seem  to  preclude  the  idea  of  a  slow  transgression  of  the  Knox- 
ville sea  over  an  uneven  surface  in  the  region  of  tlie  present 
Coast  Ranges;  it  suggests  rather  the  sudden  submergence  of  a 
land  surface  of  low  relief,  or,  to  put  it  in  another  way,  a  sudden 
transition  from  a  condition  of  very  advanced  aerial  erosion  to 
a  condition  of  marine  deposition  in  extensive  shallow  lagoons 
and  swamps. 


60 

At  several  places  along  the  belt  of  outcrop  of  the  Knoxville 
formation  the  characteristic  Knoxville  fossil  Aucella  piochii  has 
been  found,  and  an  ammonite  obtained  in  it  at  the  mouth  of 
Strawberry  Canyon,  at  Berkeley,  was  identified  by  T.  W. 
Stanton  as  probably  a  species  of  Hoplites.  At  the  same  locality 
a  small  Pecten  Avas  also  found,  perhaps  a  new  species.  Frag- 
ments of  Belemnites  are  common  in  the  formation,  particularly 
north  of  Berkeley,  wdiere  they  occur  in  a  calcareous  sandstone 
and  in  the  fine  pebbly  conglomerate  already  mentioned.  Near 
the  northwestern  limit  of  the  formation  as  mapped,  north  of 
Berkeley,  it  includes  a  few^  patches  of  impure  limestone  and 
some  rusty-Aveathering  pebbly  grits.  This  limestone  has  yielded 
a  number  of  fossils,  which  have  been  studied  by  F.  M.  Ander- 
son, who  identifies  them  as  forms  characteristic  of  the  upper 
Knoxville.  These  fossils  are  Ifodiola  major,  Liicina  colusa- 
ensis,  Pecten  complexicosta,  Cardinia?,  Myoconcha?,  Turbo, 
Atresias  liratus,  and  Phylloceras  onoense.  The  lower  Knox- 
ville is  here  wanting,  the  limestone  resting  directly  upon  the 
Franciscan  rocks.  Besides  these  fossils  obscure  remains  of 
plants  are  fairly  abundant  in  the  shales. 

In  the  southern  part  of  the  San  Mateo  quadrangle  two  small 
areas  of  conglomerate  are  represented  on  the  geologic  map  as 
probably  of  Knoxville  age.  This  reference  is  based  on  the 
fact  that  in  the  territory  just  south  of  the  quadrangle  a  simi- 
larly isolated  patch  of  conglomerate  contained  remains  of 
Aucella.  A  like  area  of  conglomerate  of  probable  Knoxville 
age  is  mapped  at  the  head  of  Bolinas  Lagoon  in  the  Tamalpais 
quadrangle. 

UPPER  CRETACEOUS  SERIES, 
CHICO   FORMATION. 

Oakland  conglomerate  member. — In  the  Berkeley  Hills  the 
basal  portion  of  the  Chico  formation  consists  of  a  conglomerate 
to  which  the  name  Oakland  conglomerate  member  is  applied, 
from  its  typical  exposure  at  the  city  of  Oakland.  This  con- 
glomerate outcrops  along  a  belt  paralleling  that  of  the  Knox- 
ville formation,  upon  which  it  lies  conformably.  Like  the 
Knoxville  it  increases  in  volume  from  the  northwest  toward 


61 


the  southeast,  but  the  increase  is  not  uniform.  At  the  mouth 
of  Strawberry  Canyon  the  conglomerate  has  not  been  discov- 
ered. In  the  valley  southeast  of  Claremont  Creek  it  is  exposed 
in  rather  small  outcrops,  which  are  not  mapped,  and  at  Temes- 
cal  Lake  it  is  exposed  to  a  thickness  of  perhaps  100  feet.  From 
this  point  it  may  be  traced  southeastward  ahnost  continuously 
to  Redwood  Peak,  where  it  bulges  out  to  a  thickness  of  about 
1000  feet.  Beyond  Redwood  Peak  it  diminishes  in  volume  for 
about  a  mile  to  200  or  300  feet  and  then  expands  again,  reach- 
ing a  width  of  about  1000  feet  northeast  of  Laundry  Farm. 
From  this  point  southeastward  its  thickness  ranges  from  500 
to  700  feet,  and  still  farther  southeast,  in  the  Haywards  quad- 
rangle, it  again  expands.  The  average  thickness  of  the  con- 
glomerate in  the  Concord  quadrangle  may  be  about  500  feet. 
In  the  Haywards  quadrangle  its  average  thickness  may  be 
nearer  1000  feet. 

The  conglomerate  at  many  places  shows  distinct  stratification 
and  the  strata  dip  uniformly  to  the  northeast,  beneath  the  over- 
lying sandstones  and  shales  of  the  Chico  formation,  at  angles 
ranging  from  45°  to  65°.  The  conglomerate  exhibits  none  of 
the  subordinate  plication  observed  in  the  underlying  shale  of 
the  Knoxville  formation,  a  fact  due  to  the  more  massive  char- 
acter of  its  strata  and  their  competence  to  propagate  the  stresses 
to  which  the  region  was  subjected.  Here  and  there  beds  of  coarse 
sandstone  are  intercalated  with  the  conglomerate,  but  the  mem- 
ber as  a  whole  is  distinctly  conglomeratic.  Its  constituent 
pebbles  are  very  much  waterworu  and  rounded  and  range 
in  si/e  from  the  dimensions  of  a  marble  to  those  of  a  man's 
head.  This  conglomerate  rests  conformably  u])on  the  soft 
shales  of  the  Knoxville  formation  and  its  stratigraphic  position 
is  significant  of  an  important  event  in  the  geologic  history  of 
the  region,  for  it  indicates  an  abrupt  change  in  the  conditions 
not  only  of  deposition  but  of  erosion.  When  it  is  recalled  that 
the  Knoxville  is  the  basal  formation  of  the  Cretaceous  system 
over  a  very  wide  extent  of  the  Coast  Ranges  the  inference  is 
that  the  region  surrounding  the  basin  of  deposition  must  have 
been  one  of  low  relief,  with  low-grade  streams  carrying  fine 
silt  rather  than  coarse  sand.     The  sudden  appearance  of  the 


62 


conglomerate  above  the  shales,  not  only  in  this  quadrangle  but 
at  several  other  localities  in  the  Coast  Ranges,  indicates  the 
encroachment  of  the  deltas  of  high-grade  streams  upon  the 
basin.  It  would  seem  clear  from  this  that  the  period  of  accu- 
mulation of  the  Knoxville  deposits  was  brought  to  a  close  by 
the  orogenic  deformation  of  the  continental  margin  of  the  basin 
in  which  tliey  were  deposited,  though  such  deformation  did  not 
cause  the  floor  of  the  basin  to  emerge  from  the  water. 

Upper  part  of  Cliico  formation. — In  the  Berkeley  Hills  the 
principal  part  of  the  Chico  formation — the  part  tliat  conform- 
ably overlies  the  Oakland  conglomerate — comprises  a  thick 
accumulation  of  sandstones  and  shales,  the  sandstones  prepon- 
derating. The  upper  beds  of  the  Oakland  conglomerate  grade 
rather  abruptly  into  the  overlying  sandstones  of  the  Chico  for- 
mation, by  transitional  beds  of  pebbly  sandstone.  The  outcrop 
of  the  sandstones  and  shales  occupies  a  broad  belt  along  the 
summit  of  the  Berkeley  Hills,  parallel  to  the  belts  occupied 
by  the  Knoxville  formation  and  the  Oakland  conglomerate. 
This  belt  widens  gradually  but  steadily  toward  the  southeast, 
its  increase  in  width  being  due  to  an  increasing  volume  of 
strata  in  this  direction.  Although  the  Chico  sandstones  and 
shales  form  the  prominent  ridges  of  the  Berkeley  Hills  these 
rocks  disintegrate  readily  under  the  weather  and  yield  a  heavy 
mantle  of  sedentary  soil,  so  that,  as  a  rule,  their  stratification 
can  be  seen  only  in  steep-walled  canyons,  w^here  observations 
show  that  though  the  strike  of  the  strata  remains  fairly  con- 
stant, the  dip  is  at  some  places  reversed,  indicating  sharp  fold- 
ing. The  strata  and  the  formation  as  a  whole  dip  in  general 
to  the  northeast  at  angles  ranging  from  30°  to  80°.  The  thick- 
ness of  the  strata  is  difficult  to  measure  on  account  of  the 
obscurity  of  the  exposures  and  the  uncertainty  as  to  repetition 
by  folding,  nearly  all  parts  of  the  formation  being  very  nuich 
alike.  It  is  estimated,  however,  that  the  part  of  the  formation 
above  the  Oakland  conglomerate  mend:)er  has  a  thickness  of 
about  2000  feet  at  the  northwest  end  of  the  belt  and  of  about 
5000  feet  at  tlie  southeast  end,  within  the  Concord  quadrangle. 
The  belt  of  outcrop  of  the  sandstones  and  shales  of  the  Chico 


6,- 


formation  widens  steadily  from  a  feather  edge  at  a  point  north 
of  Berkeley  to  over  4  miles  in  the  vicinity  of  Haywards. 

The  only  fossils  discovered  in  the  Chico  of  this  belt  are, frag- 
ments of  a  large  Inoceramus  and  a  small  Pecten,  found  in 
Strawberry  Canyon,  and  a  number  of  echinoderms  and  other 
more  obscure  fossils,  found  at  the  sandstone  quarry  in  Shepard 
Canyon. 

On  the  northern  border  of  the  Concord  quadrangle,  south 
of  Martinez,  an  area  of  sandstones  and  shales  of  the  Chico 
formation  lies  in  the  heart  of  an  anticlinal  fold,  which  is 
flanked  on  both  sides  by  synclines  of  Tertiary  strata.  The 
general  trend  of  the  axis  of  this  fold  is  northwest  and  southeast 
and  the  pitch  of  that  part  of  it  which  lies  within  the  Concord 
quadrangle  is  to  the  southeast.  The  belt  has  a  maximum 
wndth  of  2|-  miles  and  the  dips  are  prevailingly  steep,  so  that 
the  belt  seems  to  include  a  great  thickness  of  strata,  but  this 
apparent  thickness  probably  much  exceeds  the  real  thickness, 
the  volume  of  strata  being  multiplied  by  folding.  The  anti- 
cline has  not  been  eroded  deeply  enough  to  reveal  the  base  of 
the  Chico  and  the  underlying  rocks,  but  the  partial  section 
exposed  has  a  thickness  of  not  less  than  3000  feet.  This  body 
of  strata  is  composed  almost  wholly  of  massively  bedded  sand- 
stone but  includes  subordinate  beds  of  shale  and  one  lens  of 
conglomerate.  The  sandstone  is  of  medium  fine  texture,  is 
well  cemented,  and  has  a  bluish-gray  color.  Under  weather- 
ing, however,  it  becomes  rather  incoherent  and  crumbling, 
takes  on  a  tawny  color,  and  affords  an  abundant  soil,  so  that 
fresh  faces  of  the  rock  are  rarely  seen. 

Fossils. — Numerous  characteristic  Chico  fossils  have  been 
found  in  these  beds.  The  locality  within  the  area  here 
described  tliat  has  yielded  them  most  abundantly  is  in  the 
upper  part  of  the  section  exposed  on  the  east  side  of  the  Arroyo 
del  Hambre,  about  three-quarters  of  a  mile  south  of  the  north- 
ern border  of  the  Concord  quadrangle.  The  more  important 
fossils  found  here,  as  determined  by  Prof.  J.  C.  Merriam,  are  as 
follows : 


64 


Fossils  of  the  Chico  formation. 


Corbula  cultiforinis  Gabb. 
Meekia  sella  Gabb. 
Meekia  navis  Gabb. 
Meretrix  arata  or  fragilis  Gabb. 
Mytilus  quadratus  Gabb. 
Mytilus  pauperculus  Gabb. 
Nueula  tx'uncata  Gabb. 
Pecten  mai-tinezensis  Gabb. 
Pectunculus  veatchi  Gabb. 
Tellina  hoffnianniana  Gabb. 
Tellina?  aequalis  Gabb. 


Venus  varians  Gabb. 
Cinulia  obliqua  Gabb. 
Cylindrites?  brevis  Gabb. 
Dentalium  cooperi  Gabb. 
Gyrodes  expansa  Gabb. 
Perissolax  brevirostris  Gabb  n.  var. 
Pugnellus  hamulus  Gabb. 
Solarium  inornatum  Gabb. 
Helicoceras  vermicularis  Gabb. 
Sharks'  teeth. 
Teleost  fish  scale. 


TERTIARY    SYSTEM. 


EOCEKE  SERIES. 


SUBDIVISIONS. 


As  Prof.  J.  C.  Merriajii  has  clearly  shown,  the  Eocene 
rocks  of  the  California  Coast  Kanges  may  be  divided  into  two 
paleontologically  well-defined  formations  —  the  Martinez  and 
the  Tejon.  The  line  of  stratigraphic  separation  betw^een  these 
two  formations  is  not  w^ell  marked,  and,  as  the  sandstones  of 
both  yield  an  abundant  soil,  which  is  generally  cultivated,  tlie 
mapping  of  the  dividing  line  is  not  easy.  Certain  differences 
in  the  character  of  the  rocks,  however,  taken  in  connection 
with  the  fairly  frequent  occurrence  of  fossils  in  the  beds, 
facilitates  an  approximately  correct  separation,  which  has  been 
indicated  on  the  map.  These  formations  are  found  only  in 
the  northern  and  eastern  parts  of  the  Concord  quadrangle  and 
the  southern  part  of  the  San  Mateo  quadrangle. 

Along  the  northeast  flank  of  the  Berkeley  Hills  strata  of 
Miocene  age  rest  directl}^  upon  the  Chico.  Tbis  absence  of 
Eocene  rocks  in  this  part  of  the  field  is  significant  in  the 
geologic  history  of  the  region.  It  signifies  either  that  no 
Eocene  rocks  were  deposited  over  tbis  part  of  the  region,  it 
being  a  land  area  bordering  the  Eocene  basin,  or  that,  if  tliey 
were  deposited,  they  had  been  lifted  above  sea  level  by  orogenic 
movements  and  completely  removed  before  the  subsidence 
which  permitted  the  deposition  of  the  next  series  of  rocks. 
Tlie  relative  probabihty  of  these  two  interpretations  is  con- 
sidered under  tbe  heading  "Geologic  history." 


65 


Although  the  Eocene  series  is  not  found  on  the  northeast 
flank  of  the  Berkeley  Hills,  Avhere  it  should  have  been  depos- 
ited if  the  sequence  were  conformable,  it  is  abundantly  repre- 
sented on  the  flanks  of  the  anticline  south  of  ]\Iartinez, 
named  the  Franklin  anticline,  from  Franklin  Creek,  which 
runs  almost  across  it.  Here  both  the  Martinez  and  Tejon  for- 
mations are  important  elements  of  the  stratigraphy.  The  Tejon 
appears  also  in  the  core  of  a  large  anticline  northeast  of 
Sobrante  Ridge,  which  has  not  been  dissected  deeply  enough 
to  reveal  the  underlying  formations.  It  also  occurs  on  Shell 
Ridge  and  Lime  Ridge,  two  northwestern  spurs  of  Mount 
Diablo,  although  the  paleontologic  indications  at  tliese  places 
are  not  so  satisfactory,  as  Avell  as  northw^est  of  Pacheco,  where 
the  underlying  Martinez  outcrops  at  the  north  border  of  the 
quadrangle. 

MARTINEZ   FORMATION. 

Character  and  distribution. — The  Martinez  formation,  named 
from  the  town  of  Martinez,  in  Contra  Costa  County,  is,  in 
its  typical  form,  composed  of  heavily  bedded  sandstones  in 
which  much  glauconite  occurs,  giving  the  rocks  a  greenish-gray 
color.  The  formation  includes  also  some  reddish  sandstones 
and  intercalated  shales.  Roth  sandstones  and  shales  break 
down  readily  under  the  weather.  A  thick  lens  of  rather 
coarse,  well-cemented  congloine]-ate  also  occurs  in  the  south- 
easternmost  extension  of  the  terraue,  south  of  Grayson  Creek. 
These  strata  dip  away  from  the  Chico  in  apparently  conform- 
able sequence  on  both  flanks  of  the  Franklin  anticline  at 
angles  ranging  from  30°  to  80°,  the  steeper  dips  being  on  the 
northeast  flank.  The  anticline  is,  however,  complicated  by  a 
subordinate  fold  and  perhaps  by  an  axial  fault,  wdiich  brings  a 
tongue  of  the  Martinez  rocks  into  the  center  of  the  Chico  area 
and  causes  the  beds  to  dip  apparently  under  the  Chico.  The 
sections  tliat  best  reveal  the  strata  show  that  the  INIartinez  for- 
mation is  probably  not  less  tlian  2000  feet  thick.  Northeast 
of  the  Franklin  anticline  is  a  broad  syncline  of  Martinez  strata, 
in  the  trough  of  wliich  lie  folded  strata  of  the  Tejon  forma- 
tion, and  above  these,  in  the  axis  of  the  trough,  is  a  belt  of 

San  Francisco — 5 


66 


Monterey  rocks.  The  Martinez  rocks  in  the  northeast  limb 
of  this  syncline  are  exposed  in  low  hills  about  a  mile  north  of 
Pacheco.  Southwest  of  the  Franklin  anticline  the  outcrop  of 
the  Martinez  strata  on  this  limb  of  the  fold  is  cut  off  a  mile  or 
more  south  of  the  Santa  Fe  tunnel  by  a  thrust  fault  which 
causes  the  Chico  to  override  the  Monterey. 

Fossils. — Fossils  occur  at  many  places  in  the  Martinez 
formation,  most  abundantly  on  the  southwest  limb  of  the 
Franklin  anticline.  These  fossils  have  been  studied  by  Stan- 
ton, Merriam,  Weaver,  and  Dickerson,  in  whose  papers,  as  well 
as  in  earlier  papers  by  Gabb,  the  paleontology  of  the  formation 
is  fully  discussed.  The  work  of  these  paleontologists  shows 
that  the  Martinez  formation  contains  a  fauna  which  is  quite 
distinct  from  that  of  the  underlying  Chico  and  from  that  of 
the  overlying  Tejon.  So  far  as  is  now  known  only  four  of 
the  species  of  this  fauna  range  downward  into  the  Chico  and 
only  twenty-five  upward  into  the  Tejon.  Ninety-seven  species 
are  confined  to  the  Martinez.  It  has  been  even  possible  to 
separate  the  fauna  into  two  parts,  corresponding  to  an  upper 
and  lower  division  of  the  Martinez  formation.  This  distinc- 
tively Martinez  fauna  comprises  the  following  species,  the  list 
of  which  has  been  revised  by  Dr.  Dickerson." 

Characteristic  fossils  of  the  loiaer  part  of  the  Martinez  formation. 


Flabellum  remondianum  Gabb. 
Troclioeyathus  zitteli  Merriain. 
Sehizaster  leoontei  Merriam. 
Cardimn  cooperi  Gabb. 
Cuculhea  mathewsoni  Gabb. 
Leda  gabbi  Conrad. 
Lima  niultiradiata  Gabb. 
Meretrix  sp. 

Modiolus  inerriami  Weaver. 
Nucula  of.  truncata  Gabb. 
Pholadoinya  nasuta  Gabb. 
Tapes  a(f.  quadrata  Gabb. 
Tellina  martinezensis  Weaver. 


Actseon  lawsoni  Weaver. 
Cylicbna  costata  Gabb. 
Dentaliuin  cooperi  Gabb. 
Discohelix  californicus  Weaver. 
Fusus  fequilateralis  Weaver. 
Neptunea  mucrouata  Gabb. 
Perissolax  tricarinatus  Weaver. 
Siphonalia  lineata  Stanton. 
Urosyca  caudata  Gabb. 
Urosyca  robusta  W^eaver. 
Xenophora  zitteli  Weaver. 
Teredo  sp. 
Turbinella  crassatesta  Gabb. 


"Dickerson,  Roy  E.,  Fauna  of  the  Martinez  Eocene  of  California:  Cali- 
fornia Univ.  Dept.  Geo).  Bull.,  vol.  8,  No.  G,  Berkeley,  1914. 


67 


Characteruiic  fossils  of  the  upper  part  of  the  Martinez  forynat ion. 


Schizaster  lecontei  Merriain. 
Cancer  ep. 

Cardium  cooperi  Gabb. 
Cucullsea  inathewsoni  Gabb. 
Leda  gabbi  Conrad. 
Modiolus  merriami  Weaver. 
Modiolus  ornatus  Gabb. 
Dentalium  cooperi  Gabb. 
Dentalium  stramineu)n  Gabb. 
Heteroternia  gabbi  Stanton. 
Heteroternia  trochoidea  Gabb. 
Heteroternia  sp.  indet. 
Natica  6p. 

Perissolax  tricarinatus  Weaver. 
Perissolax  blakei  Gabb. 
Nucula  truncata  Gabb. 
Pholadoniya  nasuta  Gabb. 


Solen  stantoni  Weaver. 
Tellina  martin ezensis  Weaver. 
Tellina  hornii  Gabb. 
Tellina  undulifera  Gabb. 
Ampullina  cf.  striata  Gabb. 
Brachysphingus  liratus  Gabb. 
Bullinula  subglobosa  AVeaver. 
Siphonalia  lineata  Stanton. 
Architectonica  tubereulata  Weaver. 
Strepsidura  pachecoensis  Stanton. 
Tiitoniuui  pulehrum  Weaver. 
Turritella  infragranulata  Gabb. 
Turritella  pachecoensis  Stanton. 
Turritella  conica  Weaver. 
Turris  sp.  indet. 
Urosyca  caudata  Gabb. 


Rocks  of  San  Pedro  Point. — In  the  San  Mateo  quadrangle  a 
belt  of  early  Tertiary  sedimentary  beds  lies  on  the  north  flank  of 
Montara  Mountain  and  extends  from  Pilarcitos  Lake  to  the  coast 
at  San  Pedro  Point,  where  the  beds  are  well  exposed  in  cliff 
sections.  (See  PL  I.)  These  beds  were  at  one  time  assigned  to 
the  Franciscan  group,  although  they  were  recognized  as  a  pecu- 
liar facies  of  that  group  which  might  possibly  be  segregated 
from  it.  The  recent  discovery  in  these  rocks  of  a  stratum  con- 
taining poorly  preserved  remains  of  early  Tertiary  fossils  has 
led  to  their  separation  from  the  Franciscan  group  and  to  their 
tentative  assis'nment  to  the  Martinez  formation.  The  beds  con- 
sist  of  conglomerates,  coarse  arkose  sandstones  or  grits,  finer 
laminated  micaceous  sandstones,  dark  shales,  and  thin  strata  of 
limestone.  The  conglomerates  rest  directly  on  the  quartz 
diorite  ("Montara  granite"),  and  are  followed,  in  ascending 
sequence,  fey  sandstones  and  shales,  the  whole  having  an  aggre- 
gate thickness  of  several  hundred  feet.  The  beds  in  the  coast 
section  are  folded  in  at  least  two  irregular  and  rather  twisted 
synclines  and  an  intervening  anticline,  and  the  folds  are  tra- 
versed by  a  number  of  faults.  These  beds  were  probably  depos- 
ited across  the  contact  of  the  Franciscan  and  the  quartz  diorite 
and  after  being  folded  were  faulted  against  the  Franciscan  on 
their  north  side. 


68 


The  fossils  found  in  these  beds  indicate  that  they  are  of 
early  Eocene  age,  and  they  are  referred  to  the  Martinez  foi'ma- 
tion  by  R.  E.  Dickerson,  who  has  kindly  supplied  the  follow- 
ing list  of  forms  identified  by  him : 

Fossils  found  in  the  beds  at  San  Pedro  Point. 


Flabellum  sp. 

Paracyathus  (?)  sp. 

Cidaris  sp. 

Terebratulina  ef.  tejonensis  Stan- 
ton. 

Cardium  cf.  cooperi  Ciabb. 

Cucullgea  cf.  inatheAvsoni  Gabb. 

Dosinia  ef.  lawsoni  n.  sp. 

Glycimeris  sp. 

Glycimeris  cf.  veatchii  var.  major 
Stanton. 

Macroeallista  (?)  packi  n.  sp. 

Meretrix  stantoni  n.  sp. 

Modiolns  cf.  bakeri  n.  sp. 

Ofetrea  buwaldana  n.  sp. 

Phacoides  diaboli  n.  sp. 

Phacoides  quadrata  n.  sp. 

Semele  (?)  sp. 

Tapes  (?)  quadrata  Gabb. 

Teredo  sp. 

Venus  (?)  sp. 

Venerieardia  sp. 


Amauropsis  (?)  sp. 

A I  aria  sp. 

Chlorostoma  (?)  sp. 

Cylichna  costata  Gabb. 

Dentalium  sp.  striated 

Dentalium  cooperi  Gabb. 

Discohelix  sp. 

Fissurella  sp. 

Galerus  excentricus  Gabb. 

Hipponyx  sp. 

Natica  sp.  a. 

Natica  (?)  sp.  b.  spiral  lined. 

Patella  sp. 

Ringinellacf.  pinguis  Gabb. 

Spiroglyphus  (?)  sp. 

Tritonlum  iiiartinezensis  n.  sp. 

Tritonium  sp.  a. 

Turritella  sp. 

Turritella  cf.  pachecoensis  Stanton. 

Urosyca  cf.  caudata  Gabb. 

Crustacean  fragments. 


TEJON  FORMATION. 


DistribiUion  and  character. — The  beds  of  the  Tejon  forma- 
tion are  best  exposed  for  study  in  the  area  northeast  of  the 
Franklin  anticline,  where  they  lie  in  a  steeply  dipping  syncline 
and  where  their  thickness  is  not  less  than  2000  feet.  The  rocks 
are  generally  free  from  glauconite  and  are  composed  of  more 
cleanl}^  washed  sand  than  that  which  formed  the  sandstones  of 
the  Martinez  formation.  They  are  either  of  a  very  Jight  color 
or  are  stained  red  with  oxide  of  iron.  They  are  also  much 
more  strongly  cemented  and  more  resistant  to  degradation  than 
the  Martinez  rocks.  The  belt  of  Tejon  represented  by  the  two 
limbs  of  this  syncline  is  continued  on  the  southeast  side  of 
Ygnacio  Valley  in  the  two  spurs  of  Mount  Diablo  that  embrace 
the  lower  part  of  Pine  Canyon.  On  the  southern  of  these 
spurs  the  Tejon  rocks  rest  upon  and  dip  away  from  younger 
(Monterey)  rocks  on  the  limb  of  an  overturned  syncline.     On 


69 


the  northern  most  spur  the  strata  dip  at  varying  angles  to 
the  northeast.  The  rocks  are  sandstones  that  are  oenerallv  as 
hard  and  resistant  as  those  of  tlie  Pacheco  syncline,  except 
some  of  the  heavy-bedded  sandstones  southeast  of  Concord, 
which  are  highly  calcareous  and  softer.  The  only  other  local- 
ity at  which  the  Tejon  appears  is  in  the  heart  of  the  Sobrante 
anticline,  where  it  is  flanked  on  both  sides  by  strata  of  Monte- 
rey age.  The  formation  is  named  from  Fort  Tejon,  in  Kern 
County,  where  it  is  typically  developed. 

Fossils. — In  the  Tejon  formation  of  the  Concord  quadrangle 
about  43  species  of  fossils  have  been  found.  Dr.  Roy  E. 
Dickerson  has  kindly  supplied  the  following  list  as  a  result 
of  his  recent  studies  of  this  fossil  fauna  : 


Fossils  of  the 

Nummuloid  (?)  sp. 
Orbitoides  sp. 

Trochocyathus  striatus  Gabb. 
Turbinolia  n.  sp. 
Cardiuiii  cooperi  Gabb. 
Cardiuin  bi'eweri  Gabb. 
Glycimeris  sagittata  Gabb. 
Glyeimeris  cor  Gabb. 
Leda  gabbi  Conrad. 
Modiolus  ornatus  (Gabb). 
Modiolus  inerriami  (Weaver). 
Meretrix  hornii  Gabb. 
Meretrix  uvasana  Conrad. 
Meretrix  ovalis  Gabb. 
Nucula  (Acila)  n.  sp. 
Spisula  n.  sp. 
Tellina  hornii  Gabb. 
Tellina  cf.  remondii  Gabb. 
Tellina  longa  Gabb. 
Tellina  martinezensis  Weaver. 
Thracia  karquinezensis  Weaver. 
Solen  parallelus  Gabb. 


Tejon  formation. 

Solen  stantoni  Weaver. 
Venericardia  planicosta  var.  hornii 

Gabb. 
Dentaliuui  stramineum  Gabb. 
Amanropsis  alveata  (Conrad). 
Architectonica  sp. 

Ancilla(01iverato)californica  Cooper. 
Bela  cf.  clathrata  Gabb. 
Cylichna  costata  Gabb. 
(Jonus  remondii  Gabb. 
Ficopsis  remondii  Gabb. 
Megistostoma  striata  Gabb. 
Morio  tuberculatus  Gabb. 
Perissolax  blakei  (Conrad). 
Perissolax  n.  sp. 
Rinella  ciinalifera  Gabb. 
Spiroglyphiis  (?)  tejonensis  Arnold. 
Turris  monolifera  Cooper. 
Turritella  uvasana  Conrad. 
Turritella  conica  Weaver. 
Tritonium  eocenicum  Weaver. 
Tritonium  impressum  Weaver. 


Only  two  of  these  species  extend  down  into  the  Cliico  and 
eight  into  the  Martinez,  and  four  of  these  eight  are  rare  or  non- 
characteristic.  The  fauna  as  a  whole  is  therefore  quite  dis- 
tinctive for  the  Tejon,  and,  as  Merriam  has  shown,  the  sharp 
difference  between  this  fauna  and  that  of  the  Martinez  forma- 
tion is  significant  of  an  important  event  in  the  geologic  history 
of  the  region  in  Eocene  time  and  thus  warrants  the  separation 
of  the  two  formations  on  the  geologic  map. 


70 


MIOCEXE    SERIES. 

MONTEREY    GROUP. 

PETROGRAPHIG  CHARACTER. 


The  Monterey  group,  named  for  the  town  of  Monterey, 
consists  of  a  thick  mass  of  sediments  of  Miocene  age,  which 
forms  a  prominent  feature  of  the  stratigraphy  and  structure  of 
the  Coast  Ranges,  particularly  the  part  south  of  the  Bay  of 
San  Francisco.  On  the  California  coast  north  of  the  bay  the 
strata  of  this  group  have  been  found  only  in  the  vicinity  of 
Point  Reyes  and  Point  Arena.  In  the  area  here  considered 
the  group  is  represented  in  the  Concord,  Haywards,  San  Fran- 
cisco, and  Mount  Tamalpais  quadrangles,  and  although  it  does 
not  occur  in  the  San  Mateo  quadrangle  it  forms  a  well-exposed 
terrane  just  south  of  it.  The  best  sections  are  those  in  the 
Concord  quadrangle,  where  the  group  is  composed  of  two 
kinds  of  rock.  One  of  these  is  ordinary  sandstone,  generally 
of  light  color  and  not  strongly  cemented  except  where  it  con- 
tains many  fossils,  which  give  it  a  firm  bond  of  carbonate  of 
lime.  The  other  kind  of  rock  of  the  Monterey  group  embraces 
a  number  of  varieties,  all  of  which  have  been  grouped  together 
in  the  older  writings  on  California  geology  as  "bituminous 
slates."  In  this  text  "slates"  will  be  replaced  by  "shales," 
a  word  that  expresses  more  correctly  the  character  of  their 
lamination,  the  term  "bituminous  shale"  being  a  convenient 
and  fairly  expressive  designation  for  these  peculiar  and  inter- 
esting rocks. 

The  bituminous  shale  is  in  part  of  organic  origin.  Some  of 
its  beds  resemble  diatomaceous  earth;  they  are  white,  soft, 
chalky,  and  more  or  less  pulverulent.  The  microscope  shows 
that  they  contain  abundant  organic  remains,  and  chemical 
examination  shows  that  they  are  composed  chiefly  of  silica. 
Other  varieties  of  these  white,  chalky  beds  are  somewhat  harder 
and  are  harsher  to  the  touch;  they  consist  in  part  of  fine 
volcanic  ash  and  may  even  grade  into  varieties  in  which  this 
constituent  seems  to  predominate.  Though  still  remaining 
soft  and  chalky,  this  white  bituminous  shale  grades  into 
creamy  or  purplish  or  brownish  varieties.  The  color  appears 
to  be  due  wholly  to  organic  matter,  for  it  can  be  burnt  out,  the 


71 


rock  becoming  white  or  assuming  a  slightly  reddish  tint,  due  to 
the  presence  of  a  small  quantity  of  ferric  oxide.  When  heated 
on  platinum  foil  both  the  wiiite  and  the  colored  varieties  emit 
a  very  distinct  bituminous  odor.  These  rocks  generally  con- 
tain a  large  proportion  of  soluble  silica,  which  may  be  removed 
by  digestion  with  caustic  potash.  The  proportion  of  soluble 
silica  in  many  of  the  chalky  facies,  presumably  those  that  are 
richer  in  organic  remains,  averages  probably  50  per  cent.  The 
residue  after  digestion  is  an  isotropic  cloudy  substance  con- 
taining a  variable  admixture  of  angular  chips  of  doubly  refract- 
ing minerals,  chiefly  quartz  and  some  angular  fragments  of 
glass.  This  isotropic  residue  is  probably  only  in  small  part 
detrital  material  and  is  doubtless  chiefly  a  palagonitic  decom- 
position product  of  very  fine  volcanic  glass  dust. 

This  chalky  facies  of  the  bituminous  shale  may  occur  either 
in  thinly  and  evenly  laminated  beds  or  in  rather  massive  thick 
beds  that  are  traversed  by  numerous  irregular  shrinkage  joints. 
Locally  the  chalky  facies  may  pass  into  an  opaline  rock,  which 
is  dense  and  compact  but  may  be  easily  scratched  with  a  steel 
point. 

These  nondetrital  rocks,  which  were  undoubtedly  deposited 
far  from  the  continental  margin,  grade  into  clayey  and  finely 
arenaceous  varieties,  which  indicate  that  at  certain  stages  of 
their  accumulation,  or  over  certain  areas  of  Monterey  sea 
bottom,  there  were  distinct  influxes  of  detrital  material. 

A  quite  different  facies  of  the  bituminous  shale  is  prevail- 
ingly cherty  or  flinty  and  is  usually  characterized  by  very 
even  lamination.  This  laminated  rock  may  consist  wholly  of 
chert  or,  as  is  more  common,  it  may  be  composed  of  an  alter- 
nation of  thin  partings  of  soft  shale  with  beds  of  chert  that 
generally  range  in  thickness  from  1  to  3  inches.  This  facies 
is  well  displayed  in  the  band  of  bituminous  shale  which  forms 
in  part  the  crest  of  the  Berkeley  Hills  (see  PI.  V)  and  which 
presents  a  very  striking  resemblance  to  the  typical  beds  of 
radiolarian  chert  of  the  Franciscan  group  in  the  Coast  Ranges. 
Many  of  the  chert  beds  that  lie  between  the  shale  partings  are 
laminated,  though  they  show  not  the  slightest  tendency  to 
cleave  alono-  the  lamimie.     In  these  beds  and  those  in  which 


72 


the  shale  partings  are  absent  the  lamination  is  made  apparent 
by  differences  in  the  color  of  the  layers  of  chert.  The  colors 
commonly  observed  are  dull  yellow,  gray,  purplish,  and  black. 
These  cherts,  like  the  chalky  facies,  contain  some  silica  that  is 
soluble  in  caustic  potash,  but  very  much  less — not  more  than 
3  or  4  per  cent.  Traces  of  microscopic  organisms  may  be  dis- 
cerned in  some  thin  sections  of  the  cherts,  but  these  traces  do 
not  resemble  the  diatomaceous  remains  found  in  the  chalky 
shale,  and  their  character  is  indeterminate.  Nearly  all  these 
cherts,  of  whatever  color,  weather  light  yellowish  or  whitish 
and  break  down  under  the  atmosphere  into  small,  sharply 
angular  or  subcubical  fragments.  The  bituminous  matter  of 
the  rocks  is  found  chiefly  in  the  soft  shale  partings  but  may 
also  be  detected  in  the  cherts  themselves. 

These  two  chief  kinds  of  bituminous  shale,  the  chalky  and 
the  cherty,  usually  occur  at  fairly  distinct  and  separate  hori- 
zons in  the  Monterey  group,  but  in  some  places  one  grades 
into  the  other.  A  bituminous  shale  at  one  horizon  may  be 
persistently  cherty  over  a  wide  area,  whereas  a  shale  at  another 
horizon  in  the  same  deposit  may  be  as  persistently  chalky.  In 
the  Concord  quadrangle,  where  the  bituminous  shale  is  sepa- 
rated into  distinct  formations  by  intervening  formations  of 
sandstone,  one  of  the  shale  formations  may  consist  typically  of 
chert  beds  interlaminated  with  thin  partings  of  shale,  while 
another  may  be  uniformly  chalky. 

Any  account  of  the  petrography  of  the  bituminous  shales, 
even  such  a  general  account  as  is  here  attempted,  would  be 
incomplete  without  a  reference  to  certain  rock  formations  that 
are  rather  constantly  associated  with  them,  although  in  very 
subordinate  volume.  One  of  these  formations  is  a  light-colored 
or  whitish  quartzose  rock,  which  resembles  in  texture  and 
appearance  an  indurated  sandstone;  the  other  is  a  gray  com- 
pact limestone,  which  generally  weathers  ocherous  yellow  and 
in  places  contains  considerable  cherty  silica.  Both  these  rocks 
occur  rather  commonly  at  several  horizons  in  the  bituminous 
sliale  and  by  their  hardness,  texture,  mode  of  weathering,  and 
other  physical  properties  may  be  readily  discriminated  from 
both  the  clierty  and  tlie  chalky  types  of  the  shale.     They  gen- 


73 


erally  occur  in  distinct  beds  not  more  than  2  or  3  feet  thick. 
The  quartzose  beds  as  seen  in  hand  specimens  appear  to  be 
aggregates  of  vitreous  particles  througli  Avhich  consideralile 
earthy  white  material  and  some  dark  grains  are  interstitially 
distributed.  The  microscope  shows  that  the  body  of  the  rock  is 
composed  of  fragments  of  quartz,  orthoclase,  and  acidic  plagio- 
clase  and  siliceous  rocks,  the  quartz  greatly  preponderating. 
All  these  are  remarkably  angular  and  are  embedded  in  an 
amorphous  cloudy  matrix  which  resembles  the  insoluble  residue 
of  the  chalky  shales.  The  sharply  angular  fragments  of  quartz 
and  feldspar  are  very  uniform  in  size,  ranging  in  diameter  from 
about  0.12  millimeter  to  5  millimeters.  The  dark  particles  are 
not  uniform  in  composition,  but  many  of  them  are  volcanic 
glass.  This  remarkable  rock,  though  here  called  a  sandstone, 
has  rather  the  character  of  a  quartzose  tuff  than  of  a  detrital 
rock,  and  its  intercalation  in  the  cherts  and  shales,  which 
generally  include  little  deti-ital  material,  supports  this  sugges- 
tion. The  question  of  its  origin  is,  however,  left  open  for  the 
present.     No  fossils  have  been  found  in  these  rocks. 

The  limestone  is  generally  magnesian  and  is  usually  devoid 
of  fossils.  Specimens  of  rock  taken  from  certain  of  its  beds, 
however,  contain  a  small  percentage  of  phosphoric  acid,  and 
moUuscan  remains,  as  well  as  bones  of  large  cetaceans,  have 
been  found  in  it  at  some  places,  as  at  Miner's  ranch,  on  the 
east  side  of  San  Pablo  Valley,  in  the  Concord  quadrangle, 
where  the  shells  and  the  bones  occur  in  the  same  matrix, 

RELATIONS   TO   OLDER   FORMATIONS. 

The  Monterey  is  exposed  in  superposition  upon  older  rocks  at 
five  localities.  At  three  of  these  it  rests  upon  the  Tejon,  at  one 
upon  rocks  that  are  probably  of  Martinez  age,  and  at  one  upon 
the  Chico.  The  simplest  relations  are  those  exposed  in  the 
Pacheco  syncline,  where  a  fairly  symmetric  synclinal  trough 
of  Monterey  strata  with  low  southeastern  pitch  is  fianked 
on  either  side  by  Tejon  strata.  The  basal  portion  of  the 
Monterey  is  characterized  by  thick  lenses  of  conglomerate,  and 
the  sandstones  associated  with  these  lenses  contain  fossils 
which,  in  the  opinion  of  Prof.  J.  C.  Merriam,  are  characteristic 


74 


of  the  lower  Monterey.  There  is  no  apparent  structural  dis- 
cordance, but  the  basal  conglomerates  are  significant  of  an 
unconformable  relation.  On  the  northeast  side  of  Shell  Ridge 
the  Monterey  is  similarly  in  contact  with  the  Tejon,  but  with 
reversed  dips,  owing  to  the  overturning  of  the  syncline  in 
which  the  strata  are  folded.  In  the  Shell  Ridge  section  there 
is  no  structural  discordance,  but  farther  along  the  contact,  in 
Pine  Canyon,  in  the  Mount  Diablo  quadrangle,  there  are 
abundant  conglomerates  at  the  base  of  the  Monterey,  which 
again  indicate  an  unconformity.  In  the  Bear  Creek  anticline, 
in  the  northwestern  part  of  the  Concord  quadrangle,  the  lower 
Monterey  strata  rest  upon  the  Tejon,  but  here  the  structure  of 
the  Tejon  is  so  obscure  that  it  is  not  possible  to  discover 
w^hether  or  not  there  is  structural  discordance,  and  no  con- 
glomerate has  been  observed  at  the  base  of  the  Monterey. 
The  consideration  of  these  three  sections  thus  affords  evidence 
of  no  very  profound  degradation  of  the  Tejon  in  the  pre- 
Monterey  interval  of  uplift. 

In  the  section  exposed  in  the  Berkeley  Hills,  however,  about 
5  miles  southwest  of  the  Bear  Creek  anticline,  the  Monterey 
rocks  rest  directly  upon  the  Chico.  Both  Tejon  and  Mar- 
tinez are  absent.  The  Claremont  shale,  the  second  formation 
of  the  Monterey  group,  lies  almost  in  direct  contact  with  the 
Chico,  the  Sobrante  or  basal  sandstone  of  the  Monterey  group 
being  represented  by  only  a  thin  layer  of  3'ellow  incoherent 
sandstone,  mapped  with  the  Claremont.  There  are  no  conglom- 
erate beds  at  the  base  of  the  Monterey  in  this  section,  for  here 
the  Eocene  rocks,  which  are  more  than  4000  feet  thick  9  or  10 
miles  farther  northeast,  either  were  not  deposited  or,  as  seems 
more  probable,  had  been  completely  removed  in  pre-Monterey 
time.  One  justification  for  the  view  that  the  Eocene  rocks 
once  extended  over  the  region  of  the  Berkeley  Hills  and  were 
removed  in  the  Eocene-Miocene  interval  is  that  this  part  of  the 
Coast  Ranges  contains  no  representative  of  the  San  Lorenzo 
formation  (Oligocene),  which  occurs  to  the  thickness  of  2500 
feet  in  the  Santa  Cruz  quadrangle.  In  San  Lorenzo  time  the 
Berkeley  Hills  region  was  probably  a  zone  of  erosion. 


/o 


The  fifth  section  in  which  the  Monterey  is  exposed  in  super- 
position upon  older  rocks  is  at  Selby,  on  San  Pablo  Bay,  in 
the  Napa  quadrangle,  a  few  miles  north  of  the  San  Francisco 
quadrangle,  where  a  well-defined  unconformity  is  revealed  in  a 
cliff.  The  surface  upon  which  the  Monterey  rocks  rest  is  a 
marine  wave-cut  terrace,  perforated  by  many  holes  made  by 
boring  mollusks.  The  strata  in  which  this  terrace  is  cut  are 
soft  black  shales,  which  are  probably  Martinez  in  age,  for  Mar- 
tinez fossils  have  been  found  in  the  sandstones  that  adjoin  them 
on  the  north.  These  shales  have  a  southerly  dip  of  about  70°, 
and  the  surface  of  the  perforated  terrace  and  the  superimposed 
sandstones  of  the  Monterey  dip  in  the  same  direction  at  about 
60°.  It  is  evident  that  the  shales  were  elevated  above  sea 
level  and  inclined  at  about  10°  to  the  horizon  when  they  were 
truncated  to  form  the  terrace.  The  sandstone  of  the  Monterey 
contains  a  fossil  fauna  which,  in  the  opinion  of  Prof.  J.  C. 
Merriam,  is  that  of  the  middle  Monterey.  The  Tejou  is  appar- 
ently absent  here,  although  it  is  abundantly  represented  only  a 
few  miles  to  the  southeast,  along  the  strike  of  the  rocks.  The 
Monterey  sea  evidently  did  not  extend  over  this  part  of  the 
region  until  middle  Monterey  time,  and  therefore  part  of  the 
erosion  is  referable  to  early  Monterey  time.  In  general,  how- 
ever, the  historical  facts  that  are  so  clearly  manifest  at  Selby 
are  consistent  with  and  support  the  interpretation  of  the  sec- 
tion in  the  Berkeley  Hills,  where  the  Monterey  rests  directly 
upon  the  Chico. 


SUBDIVISIONS. 


In  the  Concord  and  San  Francisco  quadrangles  the  Monterey 
group  is  made  up  of  sandstones  and  bituminous  shales,  indi- 
cating an  alternation  of  deposition  along  a  shore  with  deposi- 
tion in  the  deeper  water  in  which  the  nondetrital  shale  was 
laid  down.  In  its  most  complete  section  the  bituminous  shale 
occurs  in  four  distinct  divisions  with  intervenine;  divisions  of 
sandstone.  Sandstones  occupy  the  top  and  bottom  of  the  sec- 
tion, so  that  the  Monterey  here  consists  of  nine  stratigraphic 
divisions.  This  ninefold  subdivision,  however,  is  local  and 
seems  to  represent  a  vertical  oscillation  of  the  coast  in  Monterey 


76 

time,  whereby  a  region  of  shoal  water,  in  which  detrital  mate- 
rial was  accumulating  so  abundantly  as  to  mask  any  volcanic 
or  organic  admixture,  was  four  times  depressed,  permitting  the 
accumulation  of  the  deep-water  sediments  that  were  deposited 
off  the  coast  at  this  time — sediments  in  which  organic  and  vol- 
canic material  were  greatly  in  excess  of  terrigenous  detritus. 
These  oscillations  in  Monterey  time  doubtless  also  affected  the 
deeper  off-shore  region  but  probably  did  not  change  the  char- 
acter or  interrupt  the  continuity  of  sedimentation,  so  that  the 
sequence  of  nine  subdivisions  in  the  region  east  of  the  Bay  of 
San  Francisco  may  well  be  the  chronologic  equivalent  of  a 
solid,  undivided  columnar  section  of  shale  in  Monterey  County. 
In  neighboring  territory,  however,  the  uppermost  shale  thins 
out  toward  the  south  and  finally  disappears,  so  that  the  seventh 
and  ninth  subdivisions,  which  consist  of  sandstone,  come 
together,  and  as  they  are  very  much  alike  it  is  practically 
impossible  to  map  the  boundary  between  them.  It  thus 
becomes  convenient  on  the  map  to  treat  the  uppermost  shale 
as  a  member  and  to  represent  the  two  sandstones  as  one  forma- 
tion. A  little  farther  east,  in  the  Concord  quadrangle  and  in 
parts  of  the  Mount  Diablo  quadrangle,  the  absence  of  certain 
of  the  shales  shows  that  the  deeper  water  did  not  extend  so  far 
inland,  except  at  times  of  maximum  depression,  the  Monterey 
being  there  represented  largely  by  sandy  littoi'al  deposits, 
and  the  record  of  oscillation  is  not  sufficiently  impressed  upon 
the  sediments  to  be  clearly  legible. 

Similar  evidence  of  the  oscillation  of  the  sea  bottom  in 
Monterey  time  may  be  found  in  other  portions  of  the  Coast 
Ranges.  In  Santa  Barbara  County,  for  example,  according  to 
Fairbanks,  the  lower  part  of  the  Montere}'  group  consists 
chiefly  of  gypsiferous  clays.  These  clays  and  gypsum  beds 
doubtless  represent  shallow-water  deposits  in  parts  of  the 
Pacific  Ocean  that  became  landlocked  by  uplift  and  were 
subsequently  depressed,  so  that  they  received  the  accumu- 
lation of  bituminous  shale  that  rests  upon  the  clays.  In 
the  bituminous  shales  of  Santa  Cruz  County  and  Mendocino 
County  the  continuity  of  the  shale  is  interrupted  by  strata  of 
ordinary  sandstone  and   even   pebbly  beds,  indicating  unmis- 


77 


takably  the  presence  of  shallow  water.     One  such  pebbly  bed 
occurs  in  a  shale  formation  in  the  San  Francisco  quadrangle. 

The  subdivisions  of  the  strata  of  the  Monterey  group  in  the 
San  Francisco  and  Concord  quadrangles  represent  events  in  the 
geologic  history  of  the  region  that  are  sufficiently  important  to 
entitle  each  of  them  to  receive  a  distinctive  name.  These 
names  will  flicilitate  the  discussion  of  the  local  geology  and  the 
correlation  of  the  geologic  record  in  this  region  with  similar 
records  of  oscillation  in  other  parts  of  the  Coast  Kanges ;  but 
while  the  Monterey  group  may  be  thus  divided,  according  to 
its  purely  physical  features,  into  several  parts,  and  each  may 
receive  a  formation  name,  the  group  if  considered  paleontologi- 
cally  can  be  divided  into  only  three  parts.  The  formations  of 
the  Monterey  group,  particularly  the  sandstones,  are  at  many 
localities  richly  fossiliferous,  and  a  study  of  the  stratigraphic 
distribution  of  these  fossils  by  Prof.  J.  C.  Merriam  shows  that 
the  group  contains  three  fairly  characteristic  though  not  wholly 
distinct  faunas.  The  names  of  the  stratigraphic  subdivisions; 
their  relations  to  the  paleontologic  divisions,  and  the  approxi- 
mate thickness  of  each  as  observed  in  a  section  across  the  Bear 
Creek  anticline,  in  the  Concord  and  San  Francisco  quadrangles, 
are  given  in  the  following  table: 

Subdivisions  of  Monterey  group. 


Paleontologic  sub- 
divisions. 

stratigraphic  subdivisions. 

Petrographic  character. 

Thick- 
ness. 

Upper  faunal  zone. 

Briones  sandstone 

Hercules  shale  mem- 
ber   

Briones  sandstone 

Sandstone 

Bituminous  shale 

Sandstone 

Feet. 
800 

500 
1000 

Rodeo  shale 

Hatnbre  sandstone 

Tice  shale 

Bituminous  shale 

Sandstone                  _  . 

670 
1200 

Middle  favinal  zone. 

Bituminous  shale 

Sandstone    

460 

Oursan  .sandstone 

Claremont  shale 

600 

Bituminous  shale  and 
chert  _ 

250 

Lower  faunal  zone.      Sobrante  sandstone  _ 

Sandstone          .     .. 

400 

Total  thickne.s.s,  0880  feet. 


78 


SOBRANTE  SANDSTOjSTE. 


The  largest  exposure  of  the  Monterey  group  in  the  area 
here  considered  is  in  the  broad  belt  that  traverses  the  Concord 
quadrangle  diagonally  from  northwest  to  southeast.  The 
strata  in  this  belt  have  been  folded  into  a  series  of  synclines 
and  anticlines,  locally  complicated  by  faulting,  the  dissection 
of  Avhich  has  revealed  in  their  normal  sequence  all  the  forma- 
tions of  the  group.  This  belt  also  cuts  across  the  northeast 
corner  of  the  San  Francisco  quadrangle.  In  the  axis  of  the 
Sobrante  anticline  the  basal  formation  of  the  group,  the 
Sobrante  sandstone,  named  from  Sobrante  Ridge,  rests  on 
the  Tejon  formation,  and  it  is  exposed  in  another  anticline  in 
the  valle}^  of  Pinole  Greek,  where,  however,  the  underlying 
Tejon  is  not  revealed.  The  Sobrante  sandstone  is  somewhat 
variable  in  character  but  is  prevailingly  fine  grained  and  light 
colored,  though  it  shows  local  ferruginous  staining.  Some  of 
the  beds,  however,  are  gritty  and  some  are  flaggy.  In  this 
sandstone,  near  its  base,  is  a  bed  of  white  rock  that  ranges  in 
thickness  from  a  few  inches  to  20  feet  or  more.  On  micro- 
scopic examination  this  rock  proves  to  be  a  volcanic  ash,  made 
up  cliiefl}^  of  pumiceous  glass  and  angular  fragments  of  well- 
cemented  quartz. 


FOSSILS   OF   LOWER  FAUNAL   ZONE. 


Some  of  the  beds  of  the  Sobrante  sandstone  are  richly  fossil- 
iferous,  and  b}^  means  of  these  fossils  the  Sobrante  can  be 
separated  from  the  rest  of  the  Monterey  group  to  form  the 
lowest  of  its  three  paleontoloe-ic  divisions.  A  list  of  the  most 
characteristic  fossils  of  the  formation  is  given  on  page  79 ; 
the  species  were  identified  by  Prof.  J.  C.  Merriam  and  Dr. 
Bruce  Clark.  These  fossils  are  typicid  of  the  lower  part  of 
the  Monterey  of  the  Coast  Kanges  generally. 


79 


Common  fossils  of  the  Sobrante  sandstone. 


Pelecypoda: 

Area  devincta  Conrad. 

Chioue  luathewsoni  Gabb. 

Chione  n.  sp. 

Dosinia  wbitneyi  Gabb. 

Glyeinieris  .sp. 

Leda  taphria  Dall. 

Mytilus  mathewsoni  Gabb. 

Nucula  (Acila)  n.  sp.  (?) 

Ostrea  titan  Conrad. 

Pecten  n.  sp. 

Panopea  generosa  Gould. 

Phacoides  acutilineatus  Conrad. 

Psaminobia  n.  sp. 

Solen  Curtis  Gould. 

Spisulaoceidentalis  Gabb. 

Tellina  cf.  congesta  Conrad. 

Tivela  n.  sp. 

Tellina  n.  sp. 

Yoldia  impressa  Conrad. 

Thraeia  cf.  trapezoides  Conrad. 
Gastropoda: 

Agasoma  gravida  Gabb. 

Ancillaria  lisbii  Gabb. 

Batliylouia  keepi  Arnold. 

Crepidula  sp.  cf.  pra;rupta  Con- 
rad. 

Calyptrsea    cf.    costellata     Con- 
rad. 


Gastropoda — Continued. 

Calliostoma  n.  sp. 

Cancellaria  n.  sp.  cf.  heuipbilli. 

Cancellaria  condoni  Anderson. 

Fusus  stanfordensis  Arnold. 

Gyriniiiui  niathew.soni  Gabb. 

Molopborus  biplicata  Gabb. 

Neptunaea  recurva  Gabb. 

Ocinebra  n.  sp. 

Opalia  n.  sp. 

Poly  n  ices      (Neverita)      eallosa 
Gabb. 

Polynices  n.  sp. 

Scalaria  n.  sp. 

Terebra  cooperi  Anderson. 

Turritella  aff.  ocoyana  Coni'ad. 

Turritella  variata?  Conrad. 

Tliais  (Micella)  prsecursor  Dall. 
Scaphopoda: 

Dentaliura  n.  sp. 
Cirripedia: 

Balanus  sp. 
Cephalopoda: 

Aturia  n.  sp. 
Antbozoa: 

Reef  coral  sp. 
Vertebrata : 

Fish  scales  and  bones. 


This  fauna  is  so  different  from  that  of  the  underlying  Tejon 
that  the  two  formations  are  probably  unconformable.  The 
failure  to  recognize  a  structural  discordance  at  the  base  of  the 
Sobrante  sandstone  does  not  preclude  the  existence  of  such  a 
discordance,  for  the  exposures  of  the  underlying  Tejon  do  not 
fully  reveal  the  structural  relations  of  these  rocks. 


CLAREMONT   SHALE. 


The  Claremont  shale,  which  is  named  from  Claremont  Creek, 
in  the  Concord  quadrangle,  represents  the  earliest  appearance 
of  the  bituminous  shale  in  the  Montere}^  group.  It  occurs  not 
only  in  the  Sobrante  and  Pinole  Valley  anticlines  but  appears 
in  bold  and  persistent  outcro])S  along  the  crest  and  northeastern 
edge  of  the  Berkeley  Hills.  In  the  Sobrante  anticline  it  is  in 
part  soft  and  distinctly  shaly  or  chalky  and  in  places  contains 
a  large  admixture  of  line  detrital  material,  but  in  the  Berkeley 


80 


Hills  it  is  notably  cherty,  consistiDg  of  beds  of  hard,  flinty  chert 
alternating  at  regular  intervals  with  partings  of  shale.  (See 
PI.  V.)  This  belt  in  the  Berkeley  Hills  extends  in  unbroken 
continuity  across  the  southwestern  part  of  the  Concord  quad- 
rangle and  across  the  northeast  corner  of  the  Haywards 
quadrangle.  The  bituminous  Claremont  shale  in  the  Berke- 
ley Hills  rests  upon  the  Chico  formation,  no  Eocene  strata 
intervening,  and  it  has  a  thickness  of  about  1000  feet.  The 
Sobrante  sandstone,  if  present,  is  represented  only  by  a  thin 
bed  of  yellowish  friable  sandstone,  difficult  to  observe  in  the 
field  and  too  slight  a  feature  of  the  stratigraphy  to  be  repre- 
sented on  the  map.  The  stratigraphic  relations  indicate  a 
well-marked  unconformity  between  the  Monterey  in  general 
and  the  older  rocks. 

OURSAN  SA>'DSTOJJE. 

The  Oursan  sandstone,  named  from  Oursan  Kidge,  in  the 
Concord  quadrangle,  is  a  rather  fine  grained  rock.  It  outcrops 
parallel  to  the  Claremont  shale  in  the  Sobrante  and  Pinole 
Valley  anticlines  and  flanks  the  Claremont  belt  in  the  southern 
part  of  the  Berkeley  Hills,  having  been  removed  by  erosion 
from  the  northern  part  prior  to  the  deposition  of  the  next  higher 
series  of  rocks.  It  persists  southeastward  as  a  belt  traceable 
witli  some  uncertainty  across  the  northeast  corner  of  the  Hay- 
wards  quadrangle.  In  its  northwestern  exposures  it  flanks  the 
anticline  that  extends  from  the  Concord  into  the  San  Francisco 
fpiadrangle  on  tlie  north  side  of  Pinole  Valley. 

TICE    SHALK. 

The  bituminous  shale  to  which  the  name  Tice  shale  is  here 
applied  parallels  the  Oursan  sandstone  in  both  the  Sobrante 
and  the  Pinole  Valley  anticlines  along  the  northeast  flank  of 
the  Berkeley  Hills,  in  the  southern  part  of  the  Concord  quad- 
rangle and  the  northwestern  part  of  the  Haywards  quadrangle, 
and  in  the  vicinity  of  the  town  of  Walnut  Creek.  It  is  a  per- 
sistent formation  of  bituminous  shale,  prevailingly  chalky,  in 
some  places  whitish,  in  others  pinkish  or  yellowish,  and  has 
been  named  the  Tice  shale  from  its  exposures  along  Tice  Creek, 
in  the  Concord  (juadrangle. 


81 


HAMBRE  SANDSTONE. 


The  Hambre  sandstone,  named  from  the  Arroyo  del  Hambre, 
in  the  Concord  quadrangle,  has  a  wider  distribution.  Besides 
Hanking  the  Sobrante  and  Pinole  Valley  anticlines  in  parallel 
outcrop  to  the  lower  formations,  it  has  an  extensive  outcrop 
along  the  southwest  flank  of  the  Franklin  anticline,  where  it  is 
the  lowest  formation  of  the  Monterey  group  and  lies  next  to 
the  Martinez  and  Tejon  formations.  The  Hambre  sandstone 
appears  also  in  the  heart  of  the  sharply  appressed  anticline  of 
Las  Trampas  Ridge  and  in  the  axis  of  an  anticline  that  lies  north 
of  Lafayette  Ridge.  It  occurs  also  in  tlie  Walnut  Creek  syn- 
cline.  The  formation  everywhere  consists  of  medium-textured, 
slightly  ferruginous  sandstones  with  some  sandy  shales. 


RODEO   SHALE. 


The  bituminous  shale  to  which  the  name  Rodeo  shale  is  here 
applied  occurs  at  a  strongly  marked  stratigraphic  horizon  in 
the  northwest  quarter  of  the  Concord  quadrangle  and  is  also 
exposed  in  the  adjoining  part  of  the  San  Francisco  quadrangle. 
Tlie  formation  is  named  from  Rodeo  Creek,  in  the  Concord 
quadrangle.  The  shale  is  mostly  chalky  and  more  or  less 
stained  with  oxide  of  iron  but  is  locally  cherty.  Its  chief 
exposures  are  on  the  flanks  of  the  Sobrante  and  Pinole  Valley 
anticlines  and  on  the  southwest  limb  of  the  Franklin  anticline. 
It  occurs  also  in  a  narrow^  belt,  difficult  to  trace  continuously, 
wdiich  incloses  the  Hambre  sandstone  in  the  Las  Trampas 
anticline  and  appears  also  on  the  nortiieast  flank  of  the  Berkeley 
Hills  in  their  extension  across  the  Haywards  quadrangle. 


FOSSILS   OP   MIDDLE    FAUNAL   ZONE. 


The  three  formations  of  bituminous  shale,  the  Claremont,  the 
Tice,  and  tlie  Rodeo,  the  stratigraphic  position  and  distribu- 
tion of  which  have  been  briefly  indicated,  together  with  tlie 
two  separating  sandstone  formations,  the  Oursan  and  the 
Hambre,  have  sufficient  paleontologic  community  to  warrant 
their  segregation  as  a  faunal  zone  distinct  from  the  Monterey 
formations  above  and  below.  The  fossils  which  thus  distin- 
guish these  formations  in  the  middle   zone  of  the  Monterey 


San  Francisco— 6 


82 


group  are  chiefly  the  following,  the  determiDations  having  been 
made  by  Prof.  J.  C.  Merriam : 

Fossils  of  the  middle  faunal  zone. 


Pelecypoda : 

Area  devincta  Conrad. 
Chione  mathewsoni  Grabb. 
Chione  aff.  securus  Shumard. 
Chione  n.  sp 

Corbicula  cf.  duniblei  Anderson. 
Leda  coolata  Hinds. 
Hemimactra  lenticularis  Gabb. 
Mactra  n.  sp. 
Macoma  nasuta  Conrad. 
Marcia  n.  sp. 
Modiolus  rectus  Conrad. 
Panopea  generosa  Gould. 
Pandora  scapha  Gabb. 
Pecten  peckhami  Gabb. 
Pecten  andersoni  Arnold. 
Pecten  n.  sp. 


Pelecypoda — Continued. 

Phacoides  acutilineatus  Conrad. 

Psammobia  n.  sp. 

Ostrea  titan  Conrad. 

Tellina  oregonensis  Conrad. 

Tellina  n.  sp. 

Thracia  cf.  trapezoidea  Conrad. 

Solen  Curtis  Conrad. 

Tirela  n.  sp. 

Yoldia  submontereyana  Arnold. 
Gastropoda: 

Agasonia  gravida  (?)  Gabb. 

Agasoma  sanctacruzana  Arnold 
n.  var. 

Cancellaria  condoni  Anderson. 

Crepidula  sp.  indet. 

Lunatia  n.  sp. 


BRIONES   SANDSTONE. 


Character  and  distribution. — The  Briones  sandstone,  so  named 
from  the  Briones  Hills,  in  the  Concord  quadrangle,  is  the  most 
widely  distributed  formation  of  the  Monterey  group.  It  is 
prevailingly  a  light-colored  to  whitish  well-washed  sandstone, 
in  some  places  pebbly  or  conglomeratic  and  in  general  of  coarser 
texture  than  the  lower  sandstones.  Many  of  its  strata  are 
abundantly  fossiliferous  and  some  of  them  are  veritable  shell 
beds.  As  it  is  a  cleanly  washed  quartzose  sandstone  it  yields 
only  a  light  and  generally  a  thin  soil.  It  is  one  of  the  chief 
geologic  features  of  the  Concord  quadrangle,  forming  a  belt  that 
extends  diagonally  across  it  from  northwest  to  southeast.  It 
flanks  the  combined  Bear  Creek  and  Pinole  Valley  anticlines 
and  is  the  most  conspicuous  formation  of  Las  Trampas  Ridge, 
several  of  its  beds  being  hard  and  resistant  and  standing  out 
as  prominent  ridges  or  as  ribs  on  the  walls  of  the  transverse 
canyons.  This  sandstone  forms  tlie  crest  and  upper  slopes  of 
Kocky  Ridge,  one  of  the  boldest  features  of  relief  in  the  Con- 
cord quadrangle. 

Hercules  shale  member. — The  continuity  of  the  deposition  of 
the  Briones  sands  was  interrupted  in  the  northern  part  of  the 


83 


quadrangle  by  the  deposition  of  sediments  such  as  make  up 
the  bituminous  shale.  As  this  particular  deposit  of  shale  is 
nonpersistent  it  is  mapped  not  as  a  distinct  formation  but  as  a 
shale  member  of  the  Briones  sandstone  and  is  named  the  Her- 
cules shale,  from  Hercules  station,  on  San  Pablo  Bay. 


FOSSILS  OF  UPPER  FAUNAL  ZONE. 


The  fossils  listed  below,  whicli  were  identified  by  Prof.  J.  C. 
Merriam  and   Dr.  Bruce  Clark,  are  common   in   the  Briones 
sandstone  or  the  upper  faunal  zone  of  the  Monterey  group: 
Fossils  of  the  upper  faunal  zone. 


Echinodermata : 

Scutella  breweriana  R^mond. 
Pelecypoda : 

Area  trilineata  Conrad. 

Cardium    quadrigenarium   Con- 
rad. 

Cardiuni  corbis  Martin. 

Chione  securus  Shumard  n.  var. 

Cryptomya  ovalis  Conrad. 

Diplodonta  harfordi  Anderson. 

Dosinia  n.  sp.? 

Dosinia  cf.  -vvhitneyi  Gabb. 

Marcia  oregonensis  Conrad. 

Metis  alta  Conrad  n.  var. 

Macoma  secta  Conrad. 

Macoma  n.  sp. 

Modiolus  directus  Dall  {''.). 

Modiolus  rectus  Conrad. 

Modiolus  n.  sp. 

Mulinia  cf.  densata  Conrad. 

Nucula  sp. 

Pandora  scapho  Gabb. 

Panopea  genero.sa  Gould. 

Pecten  crassicardo  Conrad. 

Pecten  acutilineatus  Conrad. 


Pelecypoda — Continued. 

Ostrea  bourgeoisii  R6mond. 

Saxidomus  nuttalli  Conrad. 

Schizothserus  nuttalli  Conrad. 

Solen  sicarius  Gould. 

Solen  curtis  Conrad. 

Siliqua  lucida  Conrad. 

Spisula  albaria  Conrad. 

Spisula  catilliformis  Conrad. 

Tivela  n  sp. 

Tellina  3  sp. 

Tellina  oregonensis  Conrad. 

Yoldia  cooperi  Gabb. 
Gastropoda: 

Calyptrsea  filosa  Gabb. 

Cancellaria  vestusta  Gabb. 

Cancellaria  2  sp. 

Chrysodomus  3  sp. 

Crepidula  princeps  Conrad. 

Nassa  n.  sp. 

Neverita  recluziana  Petit  u.  vj 

Polynices  n.  sp. 

Trophon  ponderosum  Gabb. 

Trophon  n.  sp. 


PARTLY    DIFFERENTIATED    MONTEREY    STRATA    IN    THE    CONCORD 

QUADRANGLE. 

In  the  northeastern  part  of  the  Concord  quadrangle  there  are 
two  other  belts  of  strata  which  belong  to  the  Monterey  group 
and  which  appear  to  represent  more  nearly  persistent  shallow- 
water  deposition.  The  bituminous  shale  in  these  belts  is  meager 
in  amount  and  prevailingly  sandy,  so  that  in  this  area  the 
group  can  not  be  subdivided  into  formations  by  its  petrography 
as  easily  as  in  the  more  central  portion  of  the  quadrangle. 


84 


One  of  these  belts,  which  is  flanked  on  both  sides  by  the 
Tejon  formation,  lies  in  a  synclinal  trough  between  Martinez 
Ridge  and  Pacheco.  The  fossils  collected  in  this  belt  indi- 
cate that  it  includes  both  upper  and  lower  Monterey.  The 
lower  and  upper  faunal  zones  are  well  represented,  but  the 
middle  zone  is  represented  only  by  a  comparatively  thin  sandy 
bituminous  shale,  which  may  be  the  equivalent  of  the  entire 
fivefold  alternation  of  sandstone  and  bifuminous  shale  found 
a  few  miles  farther  west  or  any  part  of  it  but  which  is  mapped 
as  the  Tice  shale. 

The  second  belt  forms  the  crest  of  Shell  Ridge,  a  northwest- 
ern spur  of  Mount  Diablo,  constituting  the  northeastern  limb 
of  an  overturned  syncline.  This,  like  the  belt  near  Pacheco, 
also  apparently  represents  all  the  deposits  of  Monterey  time  but 
is  deficient  in  bituminous  shale,  including  only  one  shale  for- 
mation. The  character  of  the  rocks  in  this  belt  indicates 
that  this  part  of  the  basin  of  deposition  was  only  once  removed 
by  subsidence  far  enough  from  the  shore  to  permit  the  deeper- 
water  sedimentation  represented  by  the  bituminous  shale.  This 
body  of  bituminous  shale  has  been  mapped  as  the  Tice 
formation. 

UNDIFFERENTIATED  MONTEREY  STRATA  IN  THE  TAMALPAIS   QUADRANGLE. 

The  only  other  Monterey  strata  that  remain  to  be  noted  form 
tiie  thick  body  of  bituminous  shale  that  occupies  the  peninsular 
ridge  west  of  Bolinas  Lagoon  and  Olema  Creek,  in  the  Tamal- 
pais  quadrangle.  Neither  the  bottom  nor  the  top  of  the  Mon- 
terey group  is  here  exposed,  the  bottom  being  deeply  buried 
and  outcropping  only  in  the  quadrangle  to  the  north,  on  the 
Point  Reyes  Peninsula,  and  the  top  having  been  completely 
removed  by  erosion.  Moreover,  so  far  as  has  been  observed, 
the  shale  here  includes  no  intercalated  sandstones.  The  rocks 
consist  of  fine-grained  soft  or  chalky  shales,  which  are  locally 
strongly  bituminous,  the  bituminous  matter  being  at  some 
])laces  so  abundant  that  it  colors  the  rocks  deep  brown  or  even 
black.  In  other  places  the  shales  are  prevailingly  of  a  faded 
piirplisli  or  creamy  color.  The  lower  part  of  the  section  con- 
tains evenly  laminated  cherty  varieties  of  shale,  and  the  section 


85 


includes  also  small  concretionary  masses  of  impure  limestone, 
the  largest  2  feet  in  diameter.  The  strike  of  the  beds  is  pre- 
vailingly that  of  the  ridge  but  is  locally  complicated  by  folding, 
which  makes  it  difficult  to  estimate  the  thickness  of  the  rocks 
in  sections  normal  to  the  general  strike.  The  southern  end 
of  the  ridge  is  in  general  anticlinal,  the  beds  on  the  north- 
east side  of  the  ridge  dipping  toward  Bolinas  Bay  and  those  on 
the  southwest  side  dipping  toward  the  Pacific.  The  total 
revealed  thickness  is  probably  not  less  than  2000  feet. 

If  this  body  of  bituminous  shale  represents  all  the  shale 
formations  of  the  middle  part  of  the  Monterey  group  in  the 
Concord  quadrangle,  as  it  probably  does,  the  conditions  that 
prevailed  in  this  part  of  the  basin  of  deposition  were  the  com- 
plement of  those  that  prevailed  in  the  belts  near  Pacheco  and 
at  Shell  Ridge,  in  the  Concord  quadrangle,  for  the  sea  floor 
here  in  the  middle  of  Monterey  time  was  apparently  never 
brought  close  enough  to  the  shore  by  uplift  to  permit  an  influx 
of  littoral  sands.  The  continuity  of  bituminous-shale  deposi- 
tion in  this  region  was  thus  unbroken  and  produced  a  single 
formation,  which  corresponds  probably  to  five  or  more  forma- 
tions in  the  Concord  quadrangle. 

BASALT. 

Within  the  Santa  Cruz  quadrangle  there  are  several  masses  of 
basalt  and  diabase,  which  have  been  closely  studied  by  Haehl 
and  Arnold,"'  who  have  determined  that  these  rocks  were  the 
products  of  a  volcanic  eruption  that  occurred  in  Miocene  time. 
Small  masses  of  similar  basalts  occur  also  at  and  near  the  south- 
ern boundary  of  the  San  Mateo  quadrangle.  These  may  be 
erosional  residuals  of  flows  that  rest  u])on  the  worn  surface  of 
the  Franciscan  rocks,  but  the  relations  shown  in  structure 
sections  F-F  and  G-G  suggest  that  they  may  be  intrusive. 
They  are  of  post-Franciscan  age  and  are  doubtless  the  products 
of  volcanic  activity  of  the  period  to  which  were  assigned  the 
rocks  in  the  Santa  Cruz  quadrangle,  so  that  they  are  probably 

"Haehl,  H.  L.,  and  Arnold,  Ralph,  The  Miocene  diabase  of  the  Santa 
Cruz  Mountains  in  San  Mateo  County,  Cal. :  Am.  Philos.  Soc.  Proc,  vol.  43, 
No.  175,  p.  16.  190-4. 


86 


of  Miocene  age.  These  basalts  and  diabases  are  fully  describ(id 
by  Haehl  and  Arnold,  who  present  minute  details  as  to  their 
petrographic  character  and  geologic  relations  in  the  Santa  Cruz 
quadrangle. 

SAN  PABLO  FORMATION. 

General  features. — In  the  area  mapped  in  this  folio  the  San 
Pablo  formation  occurs  only  in  the  Concord  and  Haywards 
quadrangles.  The  rocks  composing  the  formation  are  of 
marine  origin  and  consist  chiefly  of  medium-grained  sand- 
stones which,  where  unoxidized  are  of  a  pronounced  blue 
color.  This  blue  sandstone  occurs  generally  in  massive  beds 
that  show  only  obscure  traces  of  bedding  and  weathers  typi- 
cally in  very  rugged  outcrops  that  have  a  more  or  less  cavernous 
appearance.  In  some  parts  of  the  formation  the  sandstone  is 
admixed  with  volcanic  tuff",  and  at  a  few  localities  thin  beds  of 
tuff  are  intercalated  with  the  sandstones.  Certain  beds  of  the 
blue  sandstone  are  richly  fossiliferous  and  yield  a  fiuina  that 
is  different  from  that  of  the  Monterey,  on  which  the  formation 
rests,  and  from  that  of  the  distinctly  Pliocene  formations  of  the 
region.  The  same  assemblage  of  fossils  occurs  in  other  beds  of 
sandstone  that  is  regularly  stratified,  that  is  not  blue,  and  that 
can  not  easil}^  be  distinguished  from  the  sandstones  of  the 
Monterey  group. 

Disiribution. — The  San  Pablo  formation  is  rather  widely 
distributed  in  the  Concord  quadrangle,  lying  in  several  belts. 
One  of  these  belts  is  in  a  synclinal  trough  east  and  southeast 
of  Walnut  Creek,  a  second  is  in  a  syncline  that  extends  north- 
westward from  Walnut  Creek  to  the  northern  limits  of  the 
quadrangle  and  thence  to  San  Pablo  Bay,  and  a  third  runs 
diagonally  across  the  central  part  of  the  quadrangle,  on  the 
northeast  side  of  the  large  synclinal  trough  that  contains  the 
Orinda  formation.  This  tliird  belt  splits  into  three  parts 
toward  the  southeast,  owing  to  overthrust  faulting,  one  part 
following  the  southeast  flank  of  Las  Trampas  Kidge,  the  second 
lying  along  the  southwest  flank  of  Rocky  Kidge,  and  the  third 
passing  northeast  of  Las  Trampas  Ridge. 

In  the  vicinity  of  Walnut  Creek  the  syncline  in  which  the 
San  Pablo  formation  lies  is  sharply  appressed  and  overturned, 


87 


so  that  the  San  Pablo  strata  on  the  northeastern  limb  of  the 
fold  dip  under  the  older  Monterey  beds  at  angles  ranging 
generally  from  40°  to  60°  but  in  some  places  as  low  as  30°. 
The  average  dip  is  about  45°.  The  tliickness  of  the  forma- 
tion on  this  limb  of  the  syncline  averages  about  1400  feet. 
The  outcrop  along  this  limb  forms  the  southwestern  crest 
of  Shell  Ridge,  a  spur  of  Mount  Diablo,  and  is  the  north- 
ern limit  of  a  large  outcrop  that  encircles  Mount  Diablo  on 
the  south.  The  western  limb  of  the  syncline  outcrops  in  a 
belt  about  half  a  mile  wdde  along  Walnut  Creek,  extending 
from  the  lower  end  of  San  Ramon  Valley  to  Ygnacio  Valley. 
Here  beds  of  the  San  Pablo  dip  away  from  the  beds  of  the 
Monterey  group  at  lower  angles  than  those  displayed  by  the 
Monterey  beds.  In  the  town  of  Walnut  Creek  a  bed  of  fine- 
textured  white  volcanic  ash,  from  1  to  2  feet  thick,  occurs  in 
the  lower  part  of  the  San  Pablo  formation. 

In  the  northwestern  part  of  the  quadrangle  the  San  Pablo 
formation  occurs  as  a  series  of  outlying  synclinal  patches,  of  low 
dip,  which  possibly  lie  unconformably  upon  the  Briones  sand- 
stone. These  are  outliers  of  a  Avell-defined  synclinal  trough  of 
the  San  Pablo  which  is  well  exposed  on  the  shores  of  San 
Pablo  Bay,  in  the  Napa  quadrangle. 

The  largest  exposure  of  the  San  Pablo  formation  forms  a 
belt  that  lies  between  the  Monterey  and  the  Orinda,  on  the 
northeast  side  of  the  dominant  syncline  of  the  quadrangle. 
Southeast  of  Lafayette  this  belt  broadens  and  is  synelinally 
folded.  Its  distribution  is  determined  by  a  series  of  feults, 
which  are  shown  on  the  map. 

Thichiess. — The  thickness  of  the  San  Pablo  formation, 
which  may  be  best  determined  in  the  sections  exposed  on  the 
shores  of  San  Pablo  Bay,  in  the  Napa  quadrangle,  is  about 
1700  feet.  On  the  west  flank  of  Las  Trampas  Ridge  it  seems 
to  have  about  the  same  thickness.  In  the  Walnut  Creek  syn- 
cline about  1400  feet  of  strata  are  exposed.  At  the  north  end  of 
Las  Trampas  Ridge  and  in  the  northwest  corner  of  the  Concord 
quadrangle  only  remnants  of  synclinal  troughs  are  preserved. 

Age. — The  age  of  the  San  Pablo  formation  is  determined 
partly  by  its  superposition  upon  the  Monterey  in  relations  that 


88 


indicate  unconformity  and  partly  by  the  fossils  it  contains. 
Although  the  structural  discordance  indicated  is  apparent  at 
only  a  few  places  in  the  area  here  discussed,  it  is  clearly  evident 
in  other  parts  of  the  Coast  Ranges,  where  the  Monterey  forma- 
tions were  disturbed  and  eroded  and  the  configuration  of  the 
basins  of  deposition  was  radically  changed  Ijefore  the  sediments 
that  formed  the  San  Pablo  were  laid  down.  The  San  Pablo 
fauna  has  been  considered  Miocene  by  nearly  all  paleontolo- 
gists who  have  studied  it,  but  C.  E.  Weaver,  wlio  several  years 
ago  studied  the  fauna  in  the  middle  Coast  Ranges,  referred  it 
to  the  Pliocene  because  he  identified  with  living  forms  41 
species,  or  56  per  cent,  of  the  73  species  in  the  fauna  examined 
by  him.  It  is  interesting  to  note  that  he  found  twice  as  many 
San  Pablo  forms  that  persist  to  the  present  day  as  have  been 
discovered  in  the  Merced,  or  later  Pliocene — a  fact  that  throws 
light  on  the  vicissitudes  of  the  region  in  Tertiary  time.  Bruce 
Clark's  more  recent  and  still  unpublished  investigations  of  the 
paleontology  of  the  San  Pablo  formation  have,  however, 
greatly  reduced  the  percentage  of  living  forms  and  indicate  a 
Miocene  rather  than  Pliocene  age  for  its  fauna. 

The  Santa  oMargarita  formation  in  the  Santa  Cruz  quad- 
rangle and  the  more  southern  part  of  the  Coast  Ranges  is 
probably  the  equivalent  of  the  San  Pablo,  for  it  contains 
practically  the  same  fauna  and  its  stratigraphic  relations  are 
the  same. 

F'ossih. — The  following  list  of  fossils  has  been  revised  by 
Dr.  Bruce  Clark,  who  refers  the  fauna  to  the  Miocene  and  con- 
siders it  more  closely  related  to  the  Monterey  than  to  the 
Merced  fauna : 

Fossils  of  the  San  Fab lo  formation. 

Echinodermata:  :   Pelecypoda— Continued. 

Astrodapsis  turaidus  R6mond.  |  Caidiuiu  corbis  Martyn. 

Astrodapsis  whitneyi  R(''iiiond.  Chama  pellucida  Conrad. 

Scntella  gabbi  R6niond.  Chione  2  n.  sp. 

Astrodapsis  4  n.  sp.  i  Dosinia  3  n.  sp. 

Asteris  remondi  Gabb.  '  Diplodonta  orbella  Gould. 

Pelecypoda: 

Amiantie  2  n.  sp. 

Cardium   quadrigenariuni   Con- 


Macoiiia  4  n.  sp. 

Macroeallista  newcombiana  Car- 
penter. 


rad.  j  Mactra  n.  sp. 


89 


Fossils  of  the  San  Pablo  foj-mation— Continued. 


Pelecypoda — Continued. 
Modiolus  n.  sp. 
Mulinia  densata  Conrad. 
Miiliiiia  n.  sp. 
Mytilus  3  n.  sp. 
Paphia  stauiinea  Conrad. 
Pecten  pabloensis  Conrad. 
Peeten  crassicardo  Conrad. 
Pecten  estrellanus  Conrad  n.  var. 
Pecten  5  n.  sp. 
Petricola  n.  sp. 
Phacoides  richthofeni  Gabb. 
Phacoides  tenuisculpta  Carpen- 
ter. 
Pitaria  2  n.  sp. 
Nucula  conradi  Dall. 
Platyodon  cancellatus  Conrad. 
Sanguinolaria  alata  Gabb. 
Sanguinolaria  nuttalli  Gabb. 
Solen  Curtis  Conrad. 
Solen  sicarius  Gould. 
Spisula  catilliformis  Conrad. 
Spisala  abscissa  Gabb. 
Spisula  cf.  falcata  Gould. 
Siliqua  lucida  Conrad. 


Pelecypoda — Continued. 

Yoldia  n.  sp. 

Zirphaea  dentata  Gabb. 
Gastropoda: 

Calliostoma  2  n.  sp. 

Calyptriea  filosa  Gabb. 

Calyptriea  inornata  Gabb. 

Calyptrsea  2  n.  sp. 

Crepidula  onyx  Sowerby. 

Crepidula  n.  sp. 

Polynices  (Euspira)  2  n.  sp. 

Polynices  (ISTeverita)   recluziana 
Petit  n.  var. 

Bittium  3  sp. 

Chrysodomus  4  n.  sp. 

Heniifusus  n.  sp. 

Leptothyra  n.  sp. 

Littorina  remondi  Gabb. 

Littorina  n.  sp. 

Ocinebra  2  n.  sp. 

Priene  n.  sp. 

Ranella  n.  sp. 

Tegula  3  n.  sp. 

Trophon  carisaensis  Anderson. 

Trophon  [)onderosum  Gabb. 


PLIOCENE    SERIES. 


LEONA   RHYOLITE. 


Distribution  and  character. — The  formation  for  -which  the 
name  Leona  rhyolite  is  here  nsed  is  a  lava  that  forms  a  dis- 
continuous helt  along  the  west  front  of  the  Berkeley  Hills, 
in  the  Concord  and  Hay  wards  quadrangles,  from  Hamilton 
Gulch,  in  Berkeley,  nearly  to  Decoto,  a  distance  of  21  miles. 
It  reaches  its  maximum  width  a  little  south  of  Leona  Heights, 
in  the  Concord  quadrangle,  where  it  is  about  a  mile  and  a  half 
wide.  The  formation  is  named  from  Leona  Heights.  The 
rock  varies  in  appearance  from  place  to  place  but  is  in 
general  an  acidic  or  rhyolitic  lava,  though  it  includes  local 
masses  of  darker,  more  basic  rock,  which  in  the  field  can 
not  easily  be  segregated  from  the  general  mass.  When  fresh 
or  unweathered  the  rhyolite  is  a  light  bluish-green  compact 
rock,  feebly  porphyritic  and  studded  with  minute  crystals  of 
pyrite,  and  contains  apparently  no  ferromagnesian  silicates. 
It  appears  very  massive  and  generally  shows  no  flow  structure, 


90 

but  in  its  larger  exposures  it  is  traversed  by  rather  irregular 
joints  and  cracks.  In  certain  local  facies  it  is  amygdaloidal 
and  some  of  the  vesicles  are  drawn  out.  In  other  local  facies 
it  is  of  coarser  texture  and  has  a  granular  structure.  Under 
erosion  this  rock  forms  steep  slopes  that  are  resistant  to  degra- 
dation, so  that  its  profiles  present  a  contrast  to  those  of  the 
formations  with  which  it  is  associated.  Under  the  weather 
the  rock  disintegrates  both  mechanically  and  chemically.  The 
mechanical  alteration  yields  a  soil  charged  with  small,  sharply 
angular  fragments;  the  chemical  alteration  is  due  to  the  oxida- 
tion of  the  pyrite  to  limonite  and  hematite  and  to  the  decom- 
position of  the  silicates  of  the  rock  by  the  sulphuric  acid  formed 
by  this  oxidation.  The  general  result  is  that  the  outcrop  and 
soil  are  reddish  or  yellowish  brown,  but  in  many  prominent 
outcrops  where  the  iron  oxides  have  been  leached  out  the  rock 
is  yellowish  white.  A  microscopic  examination  of  typical  speci- 
mens of  the  least  decomposed  facies  of  the  rock  shows  that  it 
consists  of  a  microcrystalline  aggregate  of  quartz  and  feldspar 
in  which  are  embedded  a  few  small  phenocrysts  of  orthoclase, 
oligoclase  and  andesine  and,  more  rarely,  corroded  phenocrysts 
of  quartz.  It  includes  also  some  polysomatic  quartz  that  grades 
into  and  interlocks  with  the  groundmass,  but  this  may  be  sec- 
ondary. It  contains  a  few  slender  prisms  of  apatite  and  crys- 
tals of  zircon  and,  in  addition  to  the  pyrite,  some  small  crystals 
of  magnetite. 

The  decomposition  of  the  rock  forms  much  white  isotropic, 
nearly  opaque  earthy  material  in  the  groundmass,  tlirough 
which  chlorite  is  usually  disseminated.  This  material  is  prob- 
ably residual  amorphous  silica.  Tlie  chlorite  is  not  derived 
in  place  from  preexisting  ferromaguesian  silicates  but  has 
migrated  through  the  rock.  Secondary  quartz,  calcite,  and 
chlorite  fill  minute  cracks.  The  abundant  chlorite  makes  the 
rock  dark.  The  feldspars  are  cloudy  and  in  part  silicified. 
Many  of  the  crystals  of  pyrite,  both  the  cubes  and  the  pentag- 
onal dodecahedra,  are  surrounded  by  tlie  white  earthy  mate- 
rial above  mentioned.  Some  of  tbe  pyrite  occurs  partly  in 
tlie  feldspars,  which  bear  allotriomorphic  relations  to  them, 
indicating  that  the  pyrite  may  perhaps  he  an  original  constit- 


91 


uent  of  the  rock.     Other  occurrences  of  pyrite  are,  however, 
probably  of  secondary  origin. 

Chemical  composition. — Tlie  chemical  composition  of  the 
rock  is  shown  in  the  following  "analyses  of  samples  selected 
from  collections  made  near  Laundry  Farm: 

Analyses  of  Leona  rhyolite. 


SiOe  - 

TiO, . 
Al,03 

FeO.- 
MnO. 


71.00 


CaO 

BaO 

MgO 

NagO 

K,0 

P2O5 

SO3 

CO, 

FeS, 

CI 

H3O  (at  110°  C.) 
Ignition 


12.70 
.66 

2.44 


,90 


1.39 
6.45 
2.99 


.28 
1.56 


100.  37 


72.12 

.17 

11.49 

2.77 

2.30 

.10 

.94 

Trace. 

.85 

5.28 

.86 


1.15 

.03 

Trace. 

.06 

.96 


71.60 

.12 

11.93 

3.00 

3.40 

.09 

.52 


1.33 

4.62 

1.90 

,62 

,08 

.38 


99.96 


.07 
,55 


100. 21 


1.  Analyzed  by  C.  P.  Richmond  in  the  laboratory  of  the  University  of 
California. 

2  and  3.  Analyzed  by  Gf.  E.  Colby  in  the  laboratory  of  the  University  of 
California. 

The  high  content  of  silica,  the  low  content  of  lime,  and  the 
proportion  of  soda  to  potash  indicate  clearly  that  the  rock  is  a 
soda  rhyolite;  but  the  fact  that  all  the  material  is  more  or  less 
altered  precludes  the  possibility  that  these  analyses  may  repre- 
sent the  true  composition  of  the  original  rock.  Some  of  the 
ferric  oxide  in  samples  2  and  3  is  doubtless  contained  in 
hematite  produced  by  the  oxidation  of  pyrite.  Sample  1  was 
practically  free  from  pyrite.     Chemically  it  closely  resembles 


92 


the  Northbrae  rhyolite,  to  be  described  under  the  next  heading, 
but  reasons  are  there  given  why  it  is  regarded  as  a  separate 
lava  flow. 

Field  relations. — This  rhyolite  is  a  lava  which  lies  indiffer- 
ently on  the  several  formations  of  the  Franciscan  group,  on  the 
rocks  intruded  into  it,  and  on  the  Knoxville  formation.  The 
rhyolite  belt  follows  in  a  general  way  the  line  of  the  Knoxville- 
Franciscan  contact.  It  is  evident  that  before  the  extrusion  of 
the  lava  the  Cretaceous  rocks  had  been  deeply  eroded  and  in 
part  completely  stripped  from  the  underlying  Franciscan.  The 
linear  disposition  of  the  rock  is  probably  due  to  the  fact  that 
the  lava  flowed  along  a  valley  that  followed  the  contact.  At 
some  places  the  rhyolite  lies  on  a  pebbly  conglomerate  that 
probably  represents  the  river  gravels  of  the  valley  into  which 
the  lava  flowed. 

The  belt  of  rhyolite  is  in  general  coincident  with  the  fault 
zone  of  the  Haywards  rift,  and  in  this  zone  there  are  many 
minor  or  auxiliary  faults,  some  of  which  traverse  the  rhyolite, 
so  that  in  some  places  the  boundaries  of  the  formation  are 
fault  contacts.  Several  faults  cut  entirely  across  the  rhyolite, 
particularly  at  Hamilton  Gulch,  in  Berkeley,  and  near  Temescal 
Lake,  in  the  Concord  quadrangle,  and  at  San  Leandro  Can- 
yon, in  the  Haywards  quadrangle. 

Age. — The  age  of  the  rhyolite  can  not  be  determined  pre- 
cisely. Its  superposition  in  certain  places  upon  the  Knoxville 
formation  clearly  indicates  that  it  was  laid  down  after  a  post- 
Cretaceous  erosion  and  is  therefore  Tertiary  or  Quaternary. 
The  western  flanks  of  the  rhyolite  belt  are  covered  by  rather 
early  Quaternary  alluvium,  so  that  the  rhyolite  is  doubtless 
Tertiary  in  age,  but  to  what  division  of  the  Tertiary  it  belongs 
is  largely  a  matter  of  conjecture.  Analogy  with  other  localities 
would  suggest  that  it  is  of  late  Tertiary  age,  probably  Pliocene. 
Since  it  is  of  about  the  same  age  and  of  the  same  chemical 
composition  as  the  Northbrae  rhyolite,  it  might  be  regarded  as 
part  of  the  same  lava  flow  if  it  were  not  for  certain  physical 
differences,  to  be  described  later. 


93 


NORTHBftAE  RHYOLITK. 


Distribution. — On  the  western  slope  of  the  Berkeley  Hills 
north  of  Berkeley  there  are  numerous  isolated  patches  of  a 
white  rhyolite  lava,  apparently  the  remnants  of  a  flow,  the 
greater  part  of  which  has  been  removed  by  erosion.  The  area 
covered  by  these  patches  is  about  5  miles  long  and  about  1 
mile  wide,  and  the  thickness  of  the  rock  probably  nowhere 
exceeds  100  feet.  This  rock  is  named  the  Northbrae  rhyo- 
lite from  the  district  of  that  name  near  Berkeley,  in  the  San 
Francisco  quadrangle,  where  it  is  most  abundant,  and  should 
be  distinguished  from  the  Leona  rhyolite,  found  southeast  of 
Berkeley.  It  lies  indifferently  upon  the  worn  surface  of  the 
Franciscan  and  of  the  Cretaceous  formations.  Dikes  of  similar 
rock  that  cut  the  Franciscan  formations  may  have  been  com- 
posed of  the  same  molten  rock  that  formed  this  lava.  The 
rhyolite  is  overlain  by  conglomerate,  which  is  referred  to  the 
Campus  formation,  and  its  relation  to  the  Orinda  formation  at 
the  northern  end  of  the  area  indicates  that  the  Orinda  also 
overlies  it.  It  may  safely  be  regarded  as  of  pre-Orinda  or 
early  Orinda  age. 

Correlation. — Its  position  suggested  that  the  rhyolite  might 
be  the  massive  correlative  of  the  pumiceous  Pinole  tuff,  which 
farther  north  occurs  at  the  base  of  the  Orinda  formation  and 
in  Sonoma  County  at  the  base  of  the  Merced  formation.  An 
analysis  by  George  E.  Colby  of  a  sample  of  the  Pinole  tuff 
obtained  near  Cordelia  is  given  in  the  table  on  page  94  with 
analyses  of  the  Northbrae  rhyolite  made  by  Charles  Palache. 
The  comparison  does  not  support  the  suggestion  afforded  by 
stratigrapliic  considerations. 

Pctrographic  features. — The  Northbrae  rhyolite  has  been 
studied  and  described  in  detail  by  Palache."  In  some  places 
it  shows  pronounced  flow  structure,  locally  marked  by  vesicu- 
lation  and  layers  of  small  spherulites.  In  other  places  it  con- 
tains hollow  spherulites,  the  largest  several  inches  in  diameter, 
which  make  up  a  considerable  part  of  the  otherwise  glassy 
rock.     One  facies  of  the  rhyolite  is  holocrystalline  and  porphy- 

"Palaclie  Charles,  California  Univ.  JJept.  Geology  Bull.,  vol.  1,  ISo.  2, 
pp.  61-73,  1893. 


94 


ritic.  The  groundmass  is  a  fine  aggregate  of  quartz  and  feld- 
spar and  the  phenocrysts  are  corroded  quartz,  orthoclase,  and 
acidic  plagioclase.  Tiie  rock  contains  no  ferromagnesian  sili- 
cates but  in  places  includes  a  little  magnetite. 

Comparison  of  analyses  of  Pinole  tuff  and  Northhrae  rhyoUte. 


SiOs 

TiO, 

AI2O3 

FeoOa 

FeO 

MnO 

CaO 

MgO 

Na,0 

K,0 

CI 

H^OCatllO"  C.) 
Ignition  


65.40 

.55 

15.35 

2.10 

1.23 

.05 

1.12 

.60 

2.07 

3.21 

.01 

1.18 

7.00 


100. 04 


75.46 


13.18 
.91 


.95 

.10 

6.88 

1.09 


,93 


99.50 


69.85 


13.34 
.73 


.87 

Trace. 

5.58 

2.68 


I      6. 15 


99.20 


1.  Pinole  tuff  near  Cordelia. 

2.  Nortlibrae  rliyolite,  spherulitic  facies. 

3.  Northbrae  rhyolite,  glassy  facies. 

The  Northbrae  rhyolite  is  chemically  similar  to  the  Leona 
rhyolite  and  is  probably  of  about  the  same  age.  It  differs 
physically,  however,  in  showing  flow  structure,  w^ith  spheru- 
lites,  in  being  glassy  in  certain  facies,  and  in  containing  no 
pyrite,  the  presence  of  which  is  a  striking  feature  of  the  Leona 
rock.  This  physical  contrast  indicates  that  the  two  rocks  are 
not  different  parts  of  the  same  lava  flow. 

PINOLE   TUFF. 

General  featiores. — The  Pinole  tuff  occurs  only  in  small 
exposures  in  the  Concord  and  San  Francisco  quadrangles  but 
is  more  extensively  exposed  in  the  Coast  Ranges  farther  north. 
The  formation  is  named  from  the  town  of  Pinole,  on  San  Pablo 
Bay,  near  which  it  is  well  exposed.  Stratigrapliically  it  lies 
chiefly  between  the  San  Pablo  formation  and  tlie  Orinda  forma- 


95 

tion,  but  it  is  in  part  interbedded  with  the  basal  sediments  of 
the  Orinda  and  therefore  appears  to  have  been  associated  in  its 
deposition  with  the  Orinda  rather  than  with  the  San  Pablo. 
The  San  Pablo  is  a  marine  formation,  whereas  the  Pinole  tuff 
on  the  shores  of  San  Pablo  Bay,  immediately  north  of  the  San 
Francisco  quadrangle,  contains  fresh-water  fossils  and  the  bones 
of  terrestrial  mammals,  thus  allying  it  with  the  Orinda  and 
indicating  that  it  represents  the  first  deposits  laid  down  in  the 
basin  that  contained  the  Orinda  lake.  The  same  kind  of  tuff, 
moreover,  occurs  in  the  fresh-water  Orinda  formation  in 
Sobrante  Pidge,  in  the  northern  part  of  the  San  Francisco 
quadrangle  and  in  the  Mount  Diablo  quadrangle,  indicating 
that  the  volcanic  activity  which  gave  rise  to  the  materials  of  the 
tuff  persisted  into  the  early  part  of  the  Orinda  epoch.  The 
Pinole  tuff,  though  occupying  a  well-defined  stratigraphic  posi- 
tion between  the  San  Pablo  and  the  Orinda  in  several  sections, 
is  not  so  persistent  as  either  of  these  formations.  On  San  Pablo 
Bay  it  has  a  thickness  of  about  1000  feet,  but  southeast  of  this 
bay,  in  the  San  Francisco  and  Concord  quadrangles,  its  volume 
abruptly  diminishes  and  it  thins  out  and  disappears  in  the 
eastern  part  of  the  Concord  quadrangle.  It  was  evidently  laid 
down  in  a  fresh-water  basin  which  displaced,  probably  after  an 
interval  of  erosion,  the  marine  basin  of  San  Pablo  time,  so  that 
there  is  warrant  for  assuming  disturbances  at  the  close  of  the 
San  Pablo  epoch  sufficient  to  cause  an  unconformity  between 
the  San  Pablo  and  the  Pinole  formations,  although  no  angular 
discordance  between  the  tw^o  formations  has  been  detected. 

Petrograph'ic  character. — In  most  of  its  deposits  the  Pinole 
tuff  is  distinctly  stratified  and  appears  to  have  been  assorted  by 
currents  of  water.  It  consists  almost  wholly  of  whitish  or 
light-yellowish  pumice,  partly  in  fragments  ranging  in  size  from 
1  to  50  millimeters  and  partly  in  fine  dust,  the  pumiceous 
character  of  which  can  be  observed  only  by  means  of  the 
microscope.  The  general  absence  of  quartz  from  the  tuff  and 
the  fact  that  at  some  places  it  contains  fragments  of  andesite 
indicate  that  the  formation  as  a  whole  represents  the  froth  of 
an  andesitic  magma  wdiicli  was  scattered  over  a  wide  expanse 
of  country  by  violent  explosions  from  volcanoes  whose  site  is 


96 


not  yet  known  but  which  probably  lay  north  of  the  Bay  of 
San  Francisco. 

Distribution. — The  most  Extensive  exposures  of  the  Pinole 
tuff  in  the  Concord  quadrangle  are  on  the  flanks  of  the  over- 
turned syncline  that  lies  east  of  Walnut  Creek.  Here  the 
outcrop  of  the  underlying  San  Pablo  on  the  west  side  of  the 
syncline  is  paralleled  by  a  belt  of  the  tuff  for  5  miles,  and  that 
on  the  east  side  is  paralleled  by  a  similar  belt  for  about  Si- 
miles. The  tuff  on  the  west  limb  of  the  syncline  dips  away 
from  the  San  Pablo  in  the  vicinity  of  Walnut  Creek  at  an 
angle  of  about  45°,  and  that  on  the  east  limb  of  the  syncline 
dips  under  the  San  Pablo  formation  at  an  angle  of  45^.  The 
tuff  on  the  west  limb  has  a  maximum  thickness  of  probably 
100  feet.  At  the  north  end  of  this  exposure  it  is  cut  off  by 
the  alluvium  of  Ygnacio  Valley,  and  at  the  south  end  it  abuts 
abruptly  against  an  east-west  fault.  Its  maximum  thickness 
on  the  east  limb  of  the  syncline  is  perhaps  150  feet,  but  it 
thins  out  and  disappears  before  it  reaches  the  eastern  boundary 
of  the  quadrangle. 

Another  deposit  of  the  Pinole  tuff  lies  at  the  northwest  end 
of  Las  Trampas  Ridge,  where  it  forms  a  simple  open  synclinal 
trough,  having  the  San  Pablo  beneath  it  and  the  Orinda  above 
it.  Around  the  edge  of  the  syncline  the  rocks  dip  from  20° 
to  45^  toward  the  axis  of  the  fold,  which  strikes  northwest. 
The  thickness  of  the  tuff  here  does  not  exceed  50  feet. 
Between  Grizzly  Creek  and  Las  Trampas  Creek,  on  the  west 
side  of  Las  Trampas  Ridge,  the  Pinole  tuff  crops  out  with  a 
northerly  strike  and  westerly  dip  and  evidently  represents  the 
aiiticlinal  correlative  of  the  syncline  just  described,  which, 
however,  has  been  broken  by  an  axial  fault. 

In  tlie  northeast  corner  of  the  San  Francisco  quadrangle 
tliere  are  two  areas  of  the  Pinole  tuff,  one  on  each  side  of 
Sobrante  Ridge.  One  of  these  is  on  the  east  side  of  Pinole 
Valley,  where  it  presents  a  bold  outcrop  along  a  low  ridge 
al)out  a  mile  long.  The  strike  of  the  tuff  strata  here  is  a  little 
west  of  north  and  is  transverse  to  the  strike  of  the  Monterey 
formations,  Avhicb  al)ut  upon  it.  The  tuff  here  has  evidently 
been  brought  against  the  Monterey  by  a  fault  that  coincides  in 


97 


trend  with  Pinole  Valley,  so  that  the  mass  is  an  outlying 
remnant  that  has  bf^en  preserved  from  erosion.  The  outcrop 
on  the  southwest  side  of  Sobrante  Kidge  is  about  2  miles  long, 
the  strata  having  a  northwesterly  strike  and  a  low  dip  to 
the  southwest  and  being  interstratified  with  the  basal  beds 
of  the  Orinda  formation.  The  thickness  of  the  tuff  here  prob- 
ably does  not  exceed  50  feet.  Tt  is  remarkable  that  in  neither 
of  these  localities  is  there  any  trace  of  the  Ban  Pablo  formation 
between  the  Pinole  tuff  and  the  rocks  of  the  Monterey  group, 
although  the  San  Pablo  is  represented  by  about  1500  feet  of 
strata  a  few  miles  farther  north,  on  the  shores  of  San  Pablo 
Bay,  and  is  equally  well  represented  farther  southeast,  in  the 
Concord  quadrangle.  This  absence  of  the  San  Pablo  from  its 
normal  stratigraphic  horizon  indicates  either  that  the  uncon- 
formity between  the  Pinole  tuff  and  the  San  Pablo  formation 
represents  a  longer  time  than  might  be  inferred  from  their 
relations  in  other  localities  in  these  quadrangles,  the  wdiole  of 
the  San  Pablo  having  been  removed  by  erosion  before  the  tuff 
was  deposited,  or  that  the  San  Pablo  was  not  deposited  in  the 
region  of  Sobrante  Ridge  because  that  region  was  a  land  area 
in  San  Pablo  time. 

ORINDA   FORMATION. 

General  features. — The  Orinda  formation,  which  is  confined 
to  the  Concord  and  San  Francisco  quadrangles,  is  named  from 
Orinda,  in  the  Concord  quadrangle.  It  forms  a  broad  belt  that 
traverses  the  middle  of  the  Concord  quadrangle  from  southeast 
to  northwest  and  extends  across  the  northeast  corner  of. the  San 
Francisco  quadrangle.  Another  belt,  broadening  toward  the 
southeast,  wdiere  it  passes  into  the  Mount  Diablo  quadrangle, 
lies  in  the  overturned  syncline  east  and  southeast  of  Walnut 
Creek.  The  formation  consists  of  a  thick  accumulation  of 
fresh-water  beds  comprising  (1)  conglomerates  that  include 
waterworn  polygenons  pebbles,  few  larger  than  a  man's  fist, 
and  in  places  strongly  cemented;  (2)  light-colored  sandstone; 
(3)  blue,  gray,  and  brown  clay  shales;  (4)  limestones;  (5)  some 
thin  seams  of  lignite;  and  (6),  at  a  few  horizons,  thin  layers  of 
brown  decomposed  volcanic  tuff.     Farther  east,  in  the  Mount 

San  Francisfo— 7 


98 


Diablo  quadrangle,  the  formation  includes  a  few  beds  of 
pumiceous  tuff  similar  to  tlie  Pinole  tuff,  in  its  lower  part. 
Fresh-water  ostracodes  are  found  at  certain  horizons  in  the 
clay  shale  and  the  sandstone.  The  limestone  is  of  two  kinds. 
Certain  beds  are  made  up  wholly  of  remains  of  ostracodes; 
in  others  the  rock  is  dense  or  compact,  is  light  gray  or  bluish 
in  color,  and,  instead  of  ostracodes,  contains  fresh-water  mol- 
lusks,  such  as  Limn^ea,  Physa,  and  Planorbis.  This  variety 
is  generally  cherty,  and  the  chert  may  be  irregularly  dis- 
tributed through  the  limestone  or  may  be  interlaminated  with 
it.  Here  and  there  the  remains  of  mollusks  are  found  in 
the  chert.  The  limestone  beds  are  thin  and  in  the  aggregate 
constitute  probably  less  than  1  per  cent  of  the  formation. 
In  San  Pablo  Ridge,  in  the  San  Francisco  quadrangle,  toward 
t]ie  northwest  end  of  the  main  belt,  the  formation  is  from 
2000  to  2500  feet  thick,  but  ferther  southeast,  near  the  south- 
ern boundary  of  the  Concord  quadrangle,  its  thickness  increases 
to  about  6000  feet. 

Stratigraphic  relations. — The  Orinda  formation  lies  conform- 
ably upon  the  Pinole  tuff,  but  southeast  of  Walnut  Creek, 
where  the  tuff  thins  out  and  disappears,  and  elsewhere 
along  the  eastern  border  of  the  area  of  its  exposure,  it  rests 
directly  upon  the  San  Pablo  formation.  In  a  large  part  of 
its  outcrop  in  these  quadrangles,  however,  it  is  underlain  by 
neither  the  Pinole  tuff  nor  the  San  Pablo  formation,  for  it 
rests  with  marked  angular  discordance  upon  the  worn  edges  of 
either  Monterey  or  Franciscan  strata.  The  general  structure 
of  the  main  belt  of  the  Orinda  is  that  of  a  great  synclinal 
trough  witli  subordinate  folds.  One  of  these  folds  forms  a 
syncline  along  San  Pablo  Kidge  east  of  Berkeley,  in  the 
trough  of  which  lies  a  later  series  of  volcanic  rocks.  Between 
this  syncline  and  the  main  trough  of  the  Orinda  there  is  a 
low  anticline  on  the  northeast  flanks  of  San  Pablo  Ridge. 
Anotlier  subordinate  syncline,  on  the  other  side  of  the  main 
trougli,  determines  the  course  of  Pleasant  Valley,  northwest  of 
Lafayette,  and  outlying  remnants  of  this  syncline  of  Orinda 
rest  upon  the  Pinole  tuff  in  small  canoe-shaped  troughs  at 
the  northwest  end  of  Las  Trampas  Ridge. 


99 

The  Pleasant  Valley  syncliiie  is  separated  from  the  main 
trough  by  a  broad  anticline  that  pitches  southeastward  and  can 
not  be  followed  far.  In  the  southern  part  of  the  Concord 
quadrangle,  near  the  head  of  Las  Trampas  Creek,  the  main 
belt  of  the  Orinda  bifurcates  at  the  end  of  an  upthrust  fault 
block  of  Monterey  and  San  Pablo  strata  which  constitutes  the 
high  and  bold  Rocky  Ridge,  one  portion  following  the  course 
of  Bolinger  Canyon  to  the  eastern  limit  of  the  quadrangle  and 
the  other  flanking  Rocky  Ridge  on  the  southwest  and  extend- 
ing southeastward  across  the  northeast  corner  of  the  Haywards 
quadrangle. 

The  synclinal  trough  of  the  Orinda  southeast  of  Walnut 
Creek  is  a  simple  fold,  which,  with  the  underlying  strata 
already  described,  has  been  overturned  by  overthrust  pressure 
from  the  northeast,  so  that  the  Orinda  beds  on  the  northeast 
flank  of  the  syncline  dip  under  the  Pinole  tuff  at  an  angle  of 
about  ?§'.  The  strata  in  certain  low  gravel-strewn  hills  in 
the  northeast  corner  of  the  Concord  quadrangle  are  also 
referred  to  the  Orinda  as  a  result  of  studies  made  by  C.  E. 
Weaver  in  the  adjoining  Napa  quadrangle,  where  their  character 
is  better  displayed. 

Fossils. — Besides  the  ostracodes  of  the  Orinda  formation,  the 
following  fresh-water  mollusks  have  been  recorded  by  J.  G. 
Cooper^'  from  the  west  side  of  San  Pablo  Creek,  along  the  road 
from  Berkeley  to  Lafayette: 

Anodonta   uuttal liana    Lea    var.    !   Limnsea  contracosta  Cooper, 
lignitiea  Cooper.  !   Planorbis  pabloanus  Cooper. 

These  fossils  were  found  in  association  with  a  thin  seam  of 
lignite.  The  first  is  a  living  species;  the  two  others  are  extinct. 
Tlie  following  fossils,  described  by  Cooper,  were  collected  north 
of  Livermore,  from  beds  that  are  probably  the  extension  of  tiie 
Orinda  formation  across  the  Mount  Diablo  quadrangle  into  the 
Pleasanton  quadrangle. 

"Cooper,  J.  G.,  On  some  Pliocene  fresh-water  fossils  of  California: 
California  Acad.  Sci.  Proc,  2d  ser.,  vol.  4,  p.  169,  1894. 


100 


Bythinella  binneyi  Tryon. 
Carinifex  newberryi  Lea. 
Cochliopa  rowelli?  Tryon. 


Limnophysa  palustris  Linn6. 
Liiunophysa  desidiosa  Say. 
Menetus  opercularis  Grould. 


Gyraulus  vermicularis  Gould.  Phy.sa  diaphana  Tryon. 

Helix  californiensis  Lea.  j  Pisidium  occidentale  NeAvberry. 

Limnophysa  humilis  Say.  I   Pompholopsis  whitel  Call. 

The  following  vertebrate  remains,  identified  by  Prof.  J.  C. 
Merriam,  have  been  found  in  the  Orinda  formation  near  the 
upper  part  of  Las  Trampas  Creek  and  in  Bolinger  Canyon, 
in  the  Concord  quadrangle: 

Right  superior  molar  of  Hipparion  speciosum. 
Left  superior  molar  of  Hipparion  speciosum. 
Radius,  ulna,  carpus,  and  metacarpus  of  a  camel. 
Left  superior  molar  of  a  camel. 
Metapodial  of  undetermined  form. 
Pelvis  of  undetermined  form. 

Age. — The  Orinda  formation  can  with  confidence  be  assigned 
to  the  Pliocene  epoch,  for  it  lies  stratigraphically  above  the 
Pinole  tuff,  which  is  Pliocene.  Its  correlation  with  other  Plio- 
cene formations  of  the  Coast  Ranges  is,  however,  somewhat 
more  diflficult.  The  mammalian  fossils  recorded  above,  frag- 
mentary as  they  are,  indicate  older  Pliocene  having  affinities 
w^ith  the  Miocene,  and  their  consideration  has  led  to  the  opin- 
ion, expressed  in  earlier  papers,  that  the  Orinda  antedates  the 
Merced  formation  of  the  Pacific  coast,  Avhich  has  usually  been 
held  to  represent  later  Pliocene,  but  the  w^ork  of  Osmont"  in 
the  Coast  Ranges  north  of  the  Bay  of  San  Francisco  has  shown 
that  the  Sonoma  tuff  of  that  area  is  intercalated  in  the  lower 
portion  of  the  Merced.  Work  done  by  C.  E.  Weaver  in  the 
Napa  quadrangle,  the  results  of  which  are  not  yet  published, 
and  the  writer's  review  of  the  whole  field  since  Osmont's  and 
Weaver's  work  Avas  done  show  further  that  the  Sonoma  tuff  is 
probably  identical  stratigraphicall}^  as  well  as  petrographically 
with  the  Pinole  tuff,  which  underlies  the  Orinda  formation.  If 
this  is  true  the  Orinda  should  obviously  be  correlated  with  the 
INIerced  on  grounds  w^hich  are  independent  of  the  evidence 
afforded    by    the   fossils   obtained    from    tlie   two   formations. 

"Osmont,  V.  C,  A  geological  section  of  the  Coast  Ranges  north  of  the 
Bay  of  San  Francisco:  California  Univ.  Dept.  Geology  Bull.,  vol.  4,  No.  3, 
pp.  89-37,  1904. 


101 

According  to  this  view  the  Orinda  is  the  fresh-water  equivalent 
of  the  Merced,  laid  down  in  an  interior  diastrophic  trough  and 
shut  off  from  the  sea  by  a  ridge  corresponding  in  position  and 
trend  with  the  present  Berkeley  Hills. 

MERGED  FORMATION. 

General  character  and  distribution. — The  Merced  formation, 
named  for  Lake  Merced,  comprises  a  thick  accumulation  of 
marine  sediments,  which  occur  chiefly  on  the  San  Francisco 
Peninsula,  in  the  San  Mateo  quadrangle,  but  of  which  there  is 
an  outlying  patch  at  and  north  of  Bolinas,  in  the  Tamalpais 
quadrangle,  and  a  siuiilarly  isolated  deposit  at  Miramontes, 
northwest  of  Halfmoon  Bay,  in  the  San  Mateo  quadrangle. 
The  Merced  is  well  exposed  in  the  section  along  the  sea  cliffs 
between  San  Francisco  and  Mussel  Rock,  where  the  maximum 
height  of  the  cliffs  is  about  700  feet.  The  section  here  shows 
about  5800  feet  of  marine  clays,  sandy  shales,  sandstone,  fine 
pebbly  conglomerates,  and  shell  beds,  all  of  which  dip  prevail- 
ingly northeastward  at  angles  ranging  from  15°  to  75°,  though 
a  small  part  dip  more  nearly  eastward  at  lower  angles,  some  as 
low  as  5°. 

Stratigraphic  relations. — The  basement  upon  which  the 
Merced  formation  rests  in  the  vicinity  of  Mussel  Rock  is 
the  worn  surface  of  volcanic  rocks  of  the  Franciscan  group. 
Between  the  basement  volcanic  rock  and  the  basal  beds  of 
the  Merced  is  a  wedge  of  post-Franciscan  and  pre-Merced 
alluvium,  now  firmly  cemented,  composed  almost  wholly  of 
fragments  derived  from  the  underlying  volcanic  rock.  Upon 
this  ancient  alluvium  lies  a  layer  of  forest  material,  less  than 
a  foot  thick,  comprising  carbonized  wood,  bark,  matted  leaves, 
and  pine  cones  of  the  species  Finns  insignis,  and  above  this 
are  the  marine  beds.  Cones  of  the  same  species  have  been 
found  in  the  marine  beds  a  few  hundred  feet  higher  in  the 
section,  just  north  of  the  landslide  near  Mussel  Rock.  Still 
higher  are  cones  of  Psendotsuga  douglasi.  In  one  of  the  can- 
yons east  of  Mussel  Rock,  where  a  small  stream  has  cut 
down  into  the  formation,  several  trunks  of  coniferous  trees  are 
exposed,    the    wood    and    bark    being   excellently    preserved. 


102 


These  trunks  lie  in  tlie  Merced  formation,  and  a  sandstone 
bed  a  little  above  them  contains  numerous  remains  of  marine 
raollusks.  Thin  lignitic  seams  also  occur  here  and  there  in 
this  section.  Toward  the  upper  part  of  the  section  there  is 
a  bed  of  white  volcanic  ash,  which  consists  chiefly  of  minute 
fragments  of  pumiceous  glass.  The  ash  contains  no  quartz 
and  is  probably  andesitic.  It  ranges  in  thickness  from  a  few 
inches  to  perhaps  2  feet.  On  the  clifls  north  of  Mussel  Rock 
the  actual  exposure  of  the  inclined  beds  measures  about  3f 
miles  along  the  shore,  or  obliquely  across  the  strike.  From 
the  cliffs  the  beds  strilvc  southeastward  along  the  southwest  side 
of  Merced  Valley  and  have  been  traced  nearly  as  far  as  San 
Mateo.  The  breadth  of  the  outcrop  along  this  belt  decreases 
steadily  to  the  southeast  by  the  disappearance  of  the  upper  ])eds. 
Everywhere,  however,  the  base  of  the  Merced  rests  upon  the 
Franciscan.  As  a  large  part  of  the  formation  is  composed  of 
soft  beds  it  is  exposed  at  but  few  places  along  the  northeastern 
side  of  the  belt,  and  it  can  with  difliculty  be  discriminated 
from  the  soft  alluvial  Quaternary  formations  and  the  sand 
dunes  that  occupy  the  adjoining  territory.  The  belt  of  Merced 
is  limited  on  the  northeast  by  a  fault  that  drops  the  formation 
not  less  than  7000  feet  against  the  Franciscan  rocks  of  San 
Bruno  Mountain.  The  actual  trace  of  this  fault  is,  how- 
ever, obscured  by  an  overlying  mantle  of  Quaternary  sands. 
Toward  the  northwest  it  passes  beneath  the  waters  of  the 
Pacific,  and  toward  the  southeast  it  passes  beneath  the  Bay  of 
San  P'rancisco. 

On  the  low  coastal  ridge  known  as  Miramontes,  northwest 
of  Pillar  Point,  in  the  southwest  corner  of  the  San  Mateo 
quadrangle,  a  remnant  of  the  Merced  forms  a  belt  about  2 
miles  long  and  a  quarter  of  a  mile  wide,  striking  northwest. 
The  rocks  of  this  belt  are  chiefly  sandstones,  sandy  shales, 
and  shell  beds,  in  part  gently  folded  in  open  anticlines  and 
synclines,  as  may  be  seen  clearly  on  the  wave-cut  terrace 
at  low  tide,  and  in  part  so  closely  appressed  that  they  stand 
nearly  vertical.  At  the  north  end  of  the  belt,  near  Seal  Cove, 
the  basal  beds  of  the  Merced  formation  rest  directly  upon 
the  granitic    rocks  that  form   the  shore  farther  north.     Here 


103 


all  the  phenomena  of  a  bowldery  beach  may  be  seen  at  the 
base  of  the  section. 

North  of  the  Golden  Gate,  at  Bolinas,  in  the  Tamalpais 
quadrangle,  another  mass  of  the  same  formation,  containing  typ- 
ical Merced  fossils,  rests  unconformably  upon  much-disturbed 
beds  of  the  Monterey  group  and  dips  eastward  beneath  Bolinas 
Bay  so  as  to  abut  upon  the  Franciscan,  against  whicli  the 
Merced  beds  have  been  faulted  by  a  northwestward  extension 
of  the  San  Bruno  fault. 

Fossils. — The  beds  of  the  Merced  formation  are  at  several 
horizons  abundantly  fossiliferous  and  have  yielded  the  forms 
named  in  the  following  list.  The  species  were  determined  by 
B.  M.  Martin,  who  has  recently  made  an  exhaustive  study  of 
the  fauna  and  who  finds  that  63  per  cent  of  the  forms  in  the 
lower  part  of  the  formation,  constituting  its  greater  part,  are 
those  of  living  species. 


Fossils  of  the  Merced  formation. 


Echinodermata : 

Scutella  interlineata  Stimpson. 
Peleeypoda : 

Area  trilineata  Conrad. 

Cardium  meekianum  Gabb. 

Cardium    quadriginorum    Con- 
rad. 

Cardium  corbis  Martyn. 

Chione  succincta  Valenciennes. 

Cryptomya  californica  Conrad. 

Macoina  inquinata  Deshayes. 

Macoma  nasuta  Conrad. 

Marcia  oregonensis  Conrad. 

Modiola  recta  Conrad. 

Mya  japoniea  Jay. 

Mytilus  edulis  Linn6. 

Nucula  superstriata  Carpenter. 

Ostrea  lurida  Carpenter. 

Pandora  grandis  Ball. 

Paphia  staleyi  Conrad. 

Paphia  staminea  Conrad. 

Paphia    staminea    var.   diversa 
Sovverby. 

Paphia  tenerrima  Carpenter. 

Pecten  sp. 

Phacoides  annulatus  Reeve. 

Cardium  centifilosum  Carpenter. 

Saxidonms  giganteus  Deshayes. 


Peleeypoda — Continued. 

Saxidomus  nuttalli  Conrad. 

Siliqua  patula  var.  nuttalli  Con- 
rad. 

Solen  sicarius  Gould. 

Spisula  albaria  Conrad. 

Spisula  catilliformis  Conrad. 

Schizothjerus  nuttalli  Conrad. 

Schizothserus    pajaroanus    Con- 
rad. 

Tellina  bodegensis  Hinds. 

Transennella  tantilla  Carpenter. 

Zirphsa  gabbii  Tryon. 
Gastropoda: 

Amphissa  corrugata  Reeve. 

Astyris  californica  Gaskoin. 

Astyris  gausapata  var.  carinada 
Reeve. 

Astyris  richthofeni  Gabb. 

Bittium  asperum  Gabb. 

Cei'ithidea  californica  Hinds. 

Chrysodomus      portolaensis? 
Arnold. 

Chrysodomus  stantoni  Arnold. 

Chrysodomus  tabulatus  Baird. 

Crepidula  grandis  ]\lidd. 

Crepidula  onyx  Sowerby. 

Crepidula  princeps  Conrad. 


104 


Fossils  of  the  Merced  formation — Continued. 


Gastropoda—  Continued. 
Drillia  inerniis  Hinds. 
Drillia  mercedensis  n.  sp. 
Lacuna  compacta  Conrad. 
Margarita  pupilla  Gould. 
Monoceros  engonatum  Conrad. 
Nassa  fossata  Conrad. 
Nassa  mendica  Gould. 
Nassa     mendica     var.     cooperi 

Forbes. 
Natica     clausa     Broderip     and 

Sowerby. 
Olivella  biplicata  Sowerby. 
Olivella  pedroana  Conrad. 


Gastropoda— Continued. 
Pachypoma  sp. 
Pisania  fortis  Carpenter. 
Pisania     fortis     var.     angulata 

Arnold. 
Ttiais  canaliculata  Duel. 
Thais  crispata  Martyn. 
Thais   crispata   var.    septentrio- 

nalis  Reeve. 
Tritonium  sp. 
Trochita  radians  Lamarck. 
Trochita  filosa  Gabb. 
Balanus  sp. 


These  fossils  establish  the  age  of  the  formation  as  late 
Pliocene.  Considered  paleoutologically,  the  upper  part  of  the 
formation  might  be  regarded  as  Quaternary,  as  it  is,  indeed, 
by  some  writers.  This  upper  part,  in  which  the  living  species 
of  mollusks  predominate,  shows  perfect  stratigraphic  continuity 
with  the  lower  part,  which  is  clearly  Pliocene,  and  the  forma- 
tion as  a  whole  antedates  the  diastrophic  movements  which 
deformed  the  region  and  ushered  in  the  Quaternary  throughout 
the  greater  part  of  California.  It  therefore  seems  best  to  use 
this  diastrophic  event  as  a  line  of  demarcation  between  the 
Tertiary  and  the  Quaternary  and  to  class  the  formation  as  a 
whole  as  Pliocene. 


BERKELEY    GROirp. 

GKJfEBAL  DESCRIPTION 


A  group  of  volcanic  lavas  and  intercalated  fluviatile  and 
lacustral  deposits  lies  on  the  southwest  side  of  tlie  main  belt 
of  the  Orinda  formation  in  the  Concord  and  San  Francisco 
quadrangles.  This  group,  which  was  originally  named  the 
Berkeleyan,  from  the  city  of  Berkeley,  near  which  the  forma- 
tions composing  it  are  well  exposed,  originally  included  the 
Orinda  (then  Orindan)  formation,  which  has  now  been  sepa- 
rated from  it,  chiefly  to  simplify  the  statement  of  geologic 
jelations,  the  Berkeley  group  being  largely  volcanic  and  lying 
unconformably  on  the  Orinda  formation,  which  consists  chiefly 
of  fresh-water  sediments.     The  l^erkeley  group  as  here  defined 


105 


comprises  three  formations,  which,  named  in  ascending  order, 
are  (1)  the  Moraga  formation,  consisting  chiefly  of  hivas  with 
subordinate  sedimentary  beds;  (2)  the  Siesta  formation,  com- 
posed of  hicustral  deposits;  and  (3)  the  Bald  Peak  basalt, 
which  includes  very  subordinate  sedimentary  intercalations. 


MORAGA  FORMATION. 


The  Moraga  formation,  so  named  from  Moraga  Valley,  in 
the  Concord  quadrangle,  consists  chiefly  of  flows  of  andesite 
and  basalt,  with  which  are  associated  some  basic  tuffs  and  beds 
of  well-cemented  rhyolite  tuff.  Between  these  volcanic  rocks 
lie  lenticular  beds  of  conglomerate,  clay,  and  limestone.  One 
of  these  limestone  beds,  which  contains  fresh-water  fossils,  is 
30  feet  thick  and  has  lavas  above  and  below  it.  This  bed  is 
well  exposed  on  the  northeast  side  of  Siesta  Valley,  the  outcrop 
extending  to  Eureka  Peak.  One  of  the  lenses  of  conglomerate 
attains  in  places  a  maximum  thickness  of  about  200  feet.  The 
earliest  flow  of  the  formation  is  a  rather  basic  amygdaloidal 
andesite,  which  shows  great  constancy  in  character  and  strong 
persistence  in  occurrence  and  which  has  a  uniform  thickness 
of  about  50  to  60  feet.  The  lavas  that  lie  above  this  flow 
are  less  persistent  in  occurrence  and  thickness  and  are  therefore 
more  lenticular  in  form.  Bather  long  intervals  evidently 
elapsed  between  the  successive  flows  of  these  lavas,  as  may  be 
judged  by  the  presence  of  brick-red  laterite  at  the  top  of 
several  of  the  lavas,  indicating  considerable  exposure  to  the 
atmosphere  before  they  were  buried  by  later  flows.  One  of 
these  intervals  was  unusually  long,  and  during  its  continuation 
the  region  was  subject  to  notable  degradation,  so  that  when 
later  lavas  were  poured  out  upon  the  surface  thus  modified 
they  filled  the  ravines  and  are  therefore  discordant  with  the 
rocks  below  them.  The  first  of  the  flows  after  this  interval 
was  the  andesite  of  Grizzly  and  Buin  peaks.  Its  discordant 
relation  to  the  lavas  and  conglomei'ate  of  the  lower  part  of 
the  formation  is  well  exposed  below  Buin  Peak,  on  the  south- 
west side  of  San  Pablo  Bidge.  The  total  thickness  of  the 
Moraga  formation  differs  from  section  to  section,  but  its  maxi- 
mum is  about  1200  feet. 


106 


SIESTA  FORMATIO]N'. 


The  lavas  of  the  Moraga  formation  that  have  just  been 
described  appear  to  have  so  greatly  interfered  with  the  drain- 
age ways  that  they  formed  an  extensive  lake  basin,  which 
lay  partly  on  the  surface  of  the  lava  and  partly  on  older  rocks. 
In  this  lake  basin  were  deposited  the  beds  of  the  Siesta  forma- 
tion, which  in  places  has  a  thickness  of  200  feet.  The  for- 
mation was  originally  named  Siestan,  from  Siesta  Valley,  in  the 
Berkeley  Hills.  It  comprises  fresh-water  sandstones  that  grade 
into  conglomerates,  clay  shales  with  seams  of  lignite,  cherty 
limestones  containing  abundant  fresh-water  fossils,  and  layers 
of  volcanic  tuff.  In  the  clays  have  been  found  the  skull  of  a 
beaver  {Sigmogompkius  lecontei),  the  teeth  of  a  species  of  Lepus, 
and  the  dentary  bone  of  a  species  of  Lacerta.  The  same  beds 
contain  remains  of  fresh-water  shells  belonging  to  the  genera 
Limnsea,  Helix,  and  Ancylus.  In  the  limestone  beds  of  the 
Siesta,  which  in  places  attain  a  thickness  of  10  to  20  feet, 
species  of  Planorbis,  Limnsea,  and  Pisidium  are  common. 
Some  of  these  fossils  are  found  in  the  chert  as  well  as  in  the 
limestone. 


BALD   PEAK  BASALT. 


The  Siesta  lacustral  epoch  was  brought  to  a  close  by  the 
flooding  of  the  lake  basin  with  basaltic  lavas  in  a  succession  of 
flows  that  are  well  exposed  on  Bald  Peak,  from  which  they 
have  been  named.  These  lavas  have  a  maximum  aggregate 
thickness  of  300  to  350  feet  and  constitute  the  summit  of  the 
Berkeley  group,  but  as  they  are  an  erosional  residual  it  is 
impossible  to  say  how  much  thicker  they  may  have  been. 
Between  two  of  these  flows  of  basalt  at  the  head  of  Siesta 
Valley  is  a  short  lens  of  fresh-water  limestone. 

RELATIONS  OF  THE   GROUP  TO  ADJACENT  FORMATIONS. 

After  the  volcanic  and  lacustral  rocks  of  the  Berkeley  group 
had  accumulated  to  a  thickness  somewhat  exceeding  their 
present  known  mass  the  region  was  disturbed  by  a  sharp 
orogenic  movement,  which  bent  the  Berkeley  and  underlying 
strata  into  a  well-marked  syncline  and  a  subordinate  anticline. 


107 


Practically  all  the  Berkeley  group  that  is  known  to-day  is  the 
remnant  of  this  synclinal  trough,  which  is  sunk  into  the 
Orinda  and  forms  a  belt  extending  from  Moraga  Valley  to 
Wildcat  Canyon,  a  distance  of  about  9  miles,  nearly  all  of  it 
in  the  Concord  quadrangle.  This  belt,  b}^  reason  of  its  hard 
volcanic  rocks,  which  adjoin  the  softer  Orinda  formation, 
makes  the  boldest  and  highest  ridges  of  the  Berkeley  Hills. 
The  two  limbs  of  the  syncline  form  parallel  ridges,  composed  of 
lavas  of  the  Moraga  formation,  and  between  them  lies  a  long 
subsequent  valley,  bottomed  for  the  most  part  by  the  clays  of 
the  overlying  Siesta  formation.  Some  remnants  of  rocks  of 
the  Berkeley  group  are  also  found  in  the  subordinate  anticline 
on  the  northeastern  flanks  of  San  Pablo  Ridge. 

The  position  of  the  Berkeley  trough  on  the  southwest  side  of 
the  Orinda  belt  and  the  fact  that  the  Orinda  is  much  thicker 
and  more  varied  in  its  strata  on  the  northeast  side  than  on  the 
southwest  indicate  that  the  Orinda  was  affected  by  crustal 
movements  before  the  lavas  of  the  Moraga  formation  were 
poured  out  and  that  the  Orinda  may  therefore  be  separated 
from  the  Berkeley  by  an  unconformity. 

TERTIARY    AND    QUATERNARY    DEPOSITS. 
SANTA  CLARA  FORMATION. 

On  the  southwest  side  of  Crystal  Springs  Lake,  in  the  San 
Mateo  quadrangle,  is  an  embankment  of  ancient  alluvium, 
which  is  well  exposed  on  the  wagon  road  to  Halfmoon  Bay. 
The  north  end  of  this  embankment  lies  a  few  hundred  yards 
northwest  of  the  road  along  the  side  of  the  lake.  This  appears 
to  be  the  northern  part  of  a  large  area  of  alluvium  that  has 
been  mapped  in  the  Santa  Cruz  folio  as  the  Santa  Clara  for- 
mation. The  embankment  at  Crystal  Springs  Lake,  which  is 
probably  300  feet  thick,  is  a  rudely  stratified  deposit  of 
angular  fragments  of  rock  and  waterworn  pebbles,  few  of  them 
larger  than  a  hen's  egg. 

The  Santa  Clara  formation  has  been  referred  in  part  on  very 
slender  evidence  to  the  late  Pliocene  and  its  beds  in  the 
Santa  Cruz  quadrangle  are  regarded  as  a  terrestrial  chronologic 
equivalent  of  the  Merced.     The  correlation  with  the  Merced 


108 


of  some  of  the  deposits  mapped  in  the  Santa  Cruz  folio  as 
the  Santa  Clara  formation  is  questionable,  however,  for  they 
consist  of  Quaternary  alluvium  and  other  distinctly  fluviatile 
and  lacustral  beds  of  undetermined  age.  The  deposit  at  Crys- 
tal Springs  Lake  closely  resembles  the  San  Antonio  forma- 
tion at  East  Oakland,  and  parts  of  the  Santa  Clara  formation 
that  lie  farther  south  are  still  more  like  the  San  Antonio 
beds,  but  the  data  necessary  for  the  geologic  correlation  of  the 
Santa  Clara  with  other  deposits  are  lacking. 

QUATERNARY    SYSTEM. 
PLEISTOCENE   SERIES. 

CAMPUS  FORMATION. 

The  Campus  formation  occurs  on  the  ridge  of  the  Berkeley 
Hills  which  separates  Strawberry  Canyon  and  the  head  of 
Wildcat  Canyon  from  the  valley  of  the  Bay  of  San  Francisco 
near  Berkeley,  in  the  San  Francisco  and  Concord  quadrangles. 
The  formation  extends  from  Strawberry  Canyon  northwest- 
ward along  this  ridge  into  Wildcat  Canyon  for  a  distance  of 
about  3  miles.  It  is  a  mile  and  a  quarter  wide  just  northeast 
of  North  Berkeley,  and  from  this  width  it  tapers  to  a  point  at 
each  end. 

The  formation  is  made  up  of  fresh-water  deposits,  fluviatile 
and  lacustral,  and  various  lavas,  tuffs,  and  agglomerates.  The 
basal  part  of  the  formation,  which  has  the  widest  distribution, 
consists  of  conglomerates  mixed  with  tuff  and  sandstones  with 
intercalated  beds  of  clay  shale  and  lenses  of  limestone.  This 
accumulation  is  interrupted  here  and  tliere  by  sheets  of  andes- 
ite  and  small  but  relatively  thick  layers  of  tuff.  Above  these 
there  is  a  series  of  beds  consisting  largely  of  tuff  but  contain- 
ing conglomerate,  lenses  of  limestone,  and  small  patches  of 
basalt.  Above  these  beds  is  a  tliick  series  of  basalt  flows,  and 
lastly,  resting  on  the  basalt,  there  are  remnants  of  a  once 
extensive  deposit  of  rhyolitic  tuffs  and  agglomerates,  now  well 
cemented. 

The  formation  occupies  a  trougli  wJiich  trends  about 
northwest  and   wdiich  lies  athwart  the  northwest  end  of  the 


109 

older  syncline  of  the  Berkeley  group — a  trough  that  is  formed 
in  part  by  a  synclinal  fold  and  in  part  by  a  fault,  the  fault 
having  dropped  the  northeast  side  of  the  trough  against 
formations  of  the  Berkeley  and  Monterey  groups.  This  fault- 
ing appears,  however,  to  have  affected  only  the  lower  mem- 
bers of  the  Campus  formation,  since  the  volcanic  rocks  of  the 
up]^er  part  of  the  formation  are  spread  out  over  the  trace  of 
the  fault  in  the  upper  part  of  Wildcat  Canyon,  in  the  Concord 
quadrangle,  and  the  displacement  must  have  occurred  during 
the  deposition  of  the  formation.  The  fault  in  general,  how- 
ever, determines  the  northeastern  boundary  of  the  trough. 
On  its  southwestern  boundary  the  basal  beds  of  the  Campus 
formation  rest  successively  upon  the  Chico,  the  Knoxville,  the 
Franciscan,  and  the  Orinda.  Although  the  lower  part  of  the 
Campus  formation  is  faulted  down  against  the  Berkeley  group, 
it  is  apparent  from  the  map  that  the  Berkeley  strata  had  been 
folded  and  in  large  measure  degraded  before  the  fresh-water 
basin  in  which  the  Campus  formation  accumulated  had  been 
formed  and  that  an  unconformity  therefore  exists  between  the 
Berkeley  and  the  Campus.  At  the  northwest  end  of  the 
trough  the  conglomerates  of  the  Campus  rest  on  the  Orinda, 
and  as  both  formations  are  of  much  the  same  character  and 
break  down  readily  into  soils  it  is  difficult  to  discriminate  the 
tw^o,  so  that  the  mapping  is  somewhat  doubtful.  The  Campus 
trou'gh  has  been  dislocated  by  a  number  of  faults  besides  the 
one  that  forms  its  northeastern  boundary. 

The  Campus  formation  is  regarded  as  Pleistocene  in  age 
because  it  lies  unconformably  upon  the  Moraga  and  Siesta 
formations  of  the  Berkeley  group,  which  is  later  than  the 
Orinda.  The  Orinda  has  been  correlated  in  time  wdth  the 
Merced  and  is  therefore  regarded  as  Pliocene.  The  Campus 
formation  Avas  originally  named  Campan,  from  the  campus  of 
the  University  of  California,  within  wdiich  portions  of  the  for- 
mation occur. 

ALAMEDA   FORMATION. 

Beneath  the  alluvial  deposits  of  Oakland  and  Berkeley  there 
is  a  formation   of  yellow   sandy  clay,   of   very   uniform   fine 


110 

texture,  which  has  been  revealed  in  numerous  excavations  and 
well  borings  in  these  cities  and  is  exposed  in  the  trench  of 
Diamond  Creek.  Without  much  change  in  character  it  passes 
into  beds  that  carry  marine  shells,  and  intercalated  with  these 
marine  deposits  are  nonpersistent  beds  of  gravel  of  fluviatile 
origin,  the  conditions  indicating  delta  formation  alternating 
with  marine  or  estuarine  deposition.  Three  wells  sunk  at  the 
corner  of  Twenty-eighth  Street  and  Thirteenth  Avenue,  East 
Oakland,  after  passing  through  the  yellow  sandy  clay  with 
some  layers  of  sand  and  gravel,  struck  blue  clay  at  a  depth  of 
about  142  feet.  This  blue  clay  was  20  feet  thick,  and  at  its 
base  were  fragments  of  marine  shells.  Below  this  bed  was  a 
layer  of  blue  sand,  10  feet  thick,  and  below  the  sand  was  clay 
that  extended  to  the  bottom  of  the  well,  190  feet.  The  same 
well  borer  found  shells  in  the  same  formation  at  a  depth  of 
110  feet  on  Eleventh  Avenue  and  "clam"  shells  at  a  depth  of 
125  feet  on  Fifth  Street  near  Clay  Street.  In  the  artesian 
wells  at  Roberts  Landing,  wdiich  pierce  chiefly  clays  and  sands 
and  some  layers  of  gravel,  marine  shells  were  found  133  feet, 
148  feet,  and  317  feet  below  the  marsh,  which  stands  very 
close  to  the  level  of  high  tide. 

The  same  formation  is  exposed  in  the  cliffs  at  the  water 
front  of  West  Berkeley,  where  it  is  a  blue  sandy  clay,  super- 
ficially and  irregularly  oxidized  brown.  This  sandy  clay 
extends  nearly  to  the  base  of  the  Berkeley  Hills  and  underlies 
the  alluvium  of  the  slope  upon  which  Berkeley  is  situated. 
In  a  well  sunk  on  the  property  of  the  late  J.  F.  Sims,  on 
Prospect  Street,  Berkeley,  immediately  at  the  l>ase  of  the  hills, 
this  clay  was  40  feet  thick.  A  light-yellowish  sand,  doubt- 
less a  phase  of  the  same  formation,  was  exposed  to  a  thick- 
ness of  about  12  feet  some  years  ago  in  excavations  made 
when  the  Mechanics  Building  was  erected  on  the  University 
campus. 

From  the  facts  narrated  it  appears  that  practically  every- 
where beneath  the  alluvial  slopes  of  Berkeley  and  Oakland  there* 
is  a  Quaternary  formation,  predominantly  of  marine  origin, 
having  a  thickness  of  several  hundred  feet.  This  formation  is 
here  called  the  Alameda  formation,  from  the  city  of  Alameda, 


Ill 


where  it  is  well  developed.  The  contrast  between  the  Alameda 
formation  and  the  overlying  alluvium  indicates  an  important 
event  in  the  geologic  history  of  the  region.  The  fine  sandy 
clay  which  everywhere  forms  the  upper  part  of  the  Alameda 
formation  extends  close  to  the  foot  of  the  steep  front  of  the 
Berkeley  Hills.  It  is  evident  that  this  steep  front  and  there- 
fore the  Berkeley  Hills  themselves  were  not  in  existence  when 
the  sandy  clay  was  deposited,  for  such  a  slope  would  be  sub- 
ject to  rapid  degradation  immediately  after  its  uplift  and  no 
products  of  this  degradation  appear  in  the  Alameda  formation. 
Between  Lake  Merced  and  the  Pacific  Ocean  in  the  San 
Mateo  quadrangle  there  is  a  deposit  of  light-yellow  sands, 
about  200  feet  thick,  which  probably  lies  unconformably  upon 
the  Merced  and  is  therefore  of  Quaternary  age  and  may  be  the 
correlative  of  the  Alameda  formation.  These  beds  are  but 
slightly  disturbed  but  can  not  easily  be  distinguished  from  the 
Merced  and  therefore  have  not  been  separately  mapped.  They 
probably  underlie  a  considerable  part  of  the  valley  southwest 
of  8an  Bruno  Mountain. 

SAN   ANTONIO    FORMATION. 

At  the  foot  of  the  steep  face  of  the  Berkeley  Hills  betw^een 
East  Oakland  and  Berkele}'^  there  is  a  great  series  of  alluvial 
fans  built  up  by  streams  that  emerge  from  the  hills.  This 
alluvial  deposit,  which  records  a  significant  chapter  in  the 
Quaternary  history  of  the  region,  is  here  called  the  San  Antonio 
formation,  from  the  township  of  that  name. 

The  alluvium  is  divisible  into  two  parts,  an  older  and  a 
younger  part,  which  are  mapped  separately.  The  older  gravel 
forms  a  belt  that  lies  closer  to  the  foot  of  the  range  and  is  the 
product  of  stream  work  done  in  the  adjoining  hills  before  the 
streams  had  cut  back  very  far  into  the  upland.  It  consists 
only  of  rock  fragments  derived  from  the  front  of  the  hills,  for 
it  contains  no  debris  from  the  prominent  band  of  chert,  of 
Monterey  age,  which  traverses  the  range  a  short  distance  back 
of  its  front  slope.  The  later  or  upper  part  of  the  alluvium 
contains  abundant  fragments  of  the  chert  and  so  re])resents  a 
later  stage  of  the  dissection  of  the   range,  at  a  period  when 


112 


the  streams  had  cut  into  the  belt  of  Monterey  rocks.  This 
younger  deposit  is  referred  to  as  the  chert-gravel  member  of 
the  formation. 

Since  its  accumulation  this  deposit  has  been  thoroughly 
dissected  and  terraced.  One  of  its  highest  points  is  in  the 
vicinity  of  Laundry  Farm,  where  its  surface  stands  250  feet 
above  sea  level.  Farther  northwest  a  broad  terrace  is  cut  out 
of  the  alluvium  at  an  altitude  of  about  175  feet  above  sea  level, 
and  below  this  is  a  flat-bottomed  valley  or  stream  terrace,  the 
upper  part  of  which  stands  at  an  altitude  of  125  feet.  This 
lower  terrace,  which  is  between  500  and  600  feet  wide,  is  fur- 
ther dissected  by  the  sharp,  narrow  trench  of  Diamond  Creek, 
about  30  feet  deep. 

The  San  Antonio  formation,  particularly  that  part  of  it 
which  lies  north  of  the  San  Francisco  quadrangle,  contains 
the  bones  of  extinct  vertebrates,  the  following  having  been 
identified : 

Morotherium  gigas  Marsh.  I     Equus  sp. 

Bison  antiquus  Leidy.  Camelid. 

Elephas  sp.  Large  carnivore,  genus  indet. 

Mastodon  americanus  Kerr.  ^chmothorus  occidentalis. 

Equus  paeificus  Leidy.  I 

A  tidewater  canal  dug  to  a  depth  of  18  feet  a  few  years  ago 
at  the  east  end  of  Alameda  exposed  13  feet  of  sand  resting  on 
5  feet  or  more  of  the  San  Antonio  formation,  in  which  were 
found  the  femur  and  pelvis  of  a  ground  sloth  {Morotherium 
gigas). 

MKRRITT   SAND. 

The  outer  edge  of  the  terraced  alluvial  embankment  of  the 
San  Antonio  formation  in  Oakland  has  a  prevailingly  steep 
front,  which  is  evidently  not  its  original  front  but  is  a  cut  cliff, 
probably  a  sea  cliff  formed  when  the  east  side  of  the  valley  of 
the  Bay  of  San  Francisco  was  60  or  70  feet  lower  than  it  is 
now.  The  lower  terrace  of  Diamond  Creek  was  the  graded 
flood  plain  of  the  stream  when  the  base  of  this  cliff  was  ai  sea 
level,  the  terrace  at  the  mouth  of  the  valley  where  it  emerges 
from  the  low  gravel  hills  having  the  same  altitude  as  the 
base  of  the   cliffs.     Marine  sediments    deposited  at  the  time 


113 


of  this  depression  now  form  the  sand  underlying  Oakland 
and  Alameda,  here  named  the  Merritt  sand,  from  its  occurrence 
on  Lake  Merritt,  in  the  city  of  Oakland.  In  a  well  sunk  on  the 
property  of  Prof.  W.  J.  Raymond,  at  the  corner  of  Grove  and 
Sixteenth  streets,  in  Oakland,  this  sand  has  a  maximum  thick- 
ness of  44  feet  and  rests  upon  blue  and  yellow  clay  having  a 
thickness  of  45  feet.  Below^  this  clay  lies  gravel,  which  was 
pierced  to  a  depth  of  6  feet  and  yielded  a  flow  of  water.  The 
Merritt  sand  is  well  exposed  in  Alameda,  wdiere  it  is  probably 
43  feet  thick,  an  estimate  made  by  assuming  that  the  highest 
part  of  Alameda  stands  about  30  feet  above  high  tide. 

TERRACE   GRAVEL. 

A  small  area  of  gravel  on  the  slope  of  Buriburi  Ridge,  in 
the  San  Mateo  quadrangle,  near  the  head  of  Crystal  Springs 
Lake,  is  one  of  several  such  small  remnants  of  stream  gravels 
left  on  terraces  of  extinct  streams.  Its  high  elevation  indicates 
that  it  is  of  Pleistocene  rather  than  Recent  age. 

RECENT    SERIES. 

TEMESCAL  FORMATION. 

The  Merritt  sand  would  naturally  grade  into  a  beach  deposit 
at  the  base  of  the  cliffs  cut  into  the  San  Antonio  formation, 
but  the  base  of  the  cliffs  is  now  everywhere  buried  under  sec- 
ondary alluvium,  which  was  derived  from  the  alluvial  embank- 
ment of  the  San  Antonio  formation  in  the  course  of  its 
degradation  and  which  consists  of  the  same  kind  of  material — 
fragments  of  the  Mesozoic  and  Tertiary  rocks  of  the  Berkeley 
Hills.  The  superposition  of  this  secondary  alluvium  upon  the 
marine  Merritt  sand  w^as  well  exposed  a  few  years  ago  b}^  a 
deep  trench  dug  on  Telegraph  A\'^ie  in  Oakland.  Just 
south  of  Hobart  Street  the  trench  was  cut  wholly  in  clean 
sand.  Nearer  Hobart  Street  the  trench  disclosed  the  feather 
edge  of  an  alluvial  deposit  wdiicli  tliickened  toward  the  north 
and  between  Hobart  and  Twenty-second  streets  rested  directly 
on  the  sand.  At  Twenty-second  Street  it  w^as  11  feet  thick, 
and  a  little  beyond  it  showed  a  thickness  of  13  feet,  the  full 
depth  of  the  trench.     This  alluvium  extends  up  to  the  foot 

San  Francisco — 8 


114 


of  the  old  sea  cliffs  and  conceals  the  underlying  beach  deposit. 
It  is  called  the  Temescal  formation,  from  the  creek  of  that 
name  in  the  San  Francisco  quadrangle,  where  it  is  well 
developed. 

The  alluvial  deposits  at  the  base  of  the  steep  slopes  of  the 
San  Francisco,  Haywards,  and  San  Mateo  quadrangles  appear 
to  be  the  chronologic  equivalents  of  the  Temescal  formation 
and  are  so  mapped. 

OTHER   RECENT  DEPOSITS. 

Terrace  deposits. — A  wave-cut  terrace  at  Bolinas  carries 
patches  of  marine  sands,  which  still  remain  on  divides  between 
the  ravines  and  gullies  that  have  been  cut  into  the  terrace  since 
its  uplift  but  which  are  not  mapped. 

Travertine. — On  the  western  slope  of  the  Berkeley  Hills 
north  of  Berkeley  there  are  sheets  of  travertine,  which  were 
deposited  by  springs  of  calcareous  water.  Most  of  these 
deposits  have  been  more  or  less  affected  by  landslides,  some 
having  been  carried  far  down  the  slope,  and  they  are  therefore 
not  shown  on  the  geologic  map.  Larger  and  thicker  deposits 
of  travertine,  which  are  mapped,  occur  on  Lime  Ridge,  a  spur 
that  projects  into  the  Concord  quadrangle  from  Mount  Diablo 
near  the  town  of  Concord. 

Dunes. — Wind-blown  sands  cover  large  areas  in  the  city  of 
San  Francisco.  These  sands  drift  in  from  the  ocean  beach 
south  of  the  entrance  to  the  Golden  Gate  and  naturally  take 
the  form  of  dunes.  Similar  dunes  occur  at  intervals  along  the 
coast  in  the  San  Mateo  quadrangle  but  are  nowhere  so  exten- 
sive as  in  San  Francisco.  Most  of  these  dunes  are  of  very 
recent  origin,  but  several  other  eolian  deposits  in  the  area  are 
evidently  much  older  and  may  be  Quaternary — such  deposits, 
for  example,  as  those  which  form  part  of  the  cliffs  on  the  south 
side  of  Hunter  Point. 

Salt-marsh  deposits. — The  only  other  deposits  that  remain  to 
be  mentioned  are  the  clays  and  silts  now  accumulating  in  the 
salt  marshes  that  fringe  the  Bay  of  San  Francisco.  The 
deposits  are  gradually  encroaching  upon  the  bay  and  tend  to 
restrict  its  area. 


115 


STRUCTURE. 
GENERAL  FEATURES. 

The  dominant  structure  of  the  region  about  the  Bay  of  Ban 
Francisco  is  expressed  in  three  long  orographic  blocks  that 
extend  from  northwest  to  southeast,  each  tilted  northeastward, 
with  its  crest  on  the  southwest  side,  as  shown  in  figures  2  (p.  10) 
and  3.    These  are  (1)  the  Montara  block,  culminating  in  Montara 


iZZ%s' 


122°30'  SAN  MATEO  y^z'lb'  HAYWARDS  122-bd 


Scale 


Figure  3.— Outline  map  of  the  Tainalpais,  San  Francisco,  Concord,  San 
Mateo,  and  Hay  wards  quadrangles,  showing  the  limits  of  the  great  fault 
blocks,  the  larger  faults,  and  the  axes  of  folds. 

The  folding  and  faultiug  occurred  at  intervals  from  the  Cretaceous  into  the  Quaternary  period. 
The  limits  of  the  major  fault  blocks  are  shown  by  heavy  daslied  lines;  faults  by  continu- 
ous lighter  lines,  except  in  the  ocean,  where  they  are  broken  ;  anticlinal  axes  by  dashed  lines ; 
synclinal  axes  by  dotted  lines.  T  indicates  the  thrust  side  of  an  overthrust  fault ;  u  the 
upraised  side  of  a  fault  block ;  D  the  downthrown  side  of  a  fault  block. 

Mountain  and  extending  northward  to  I  the  foot  of  San  Bruno 
Mountain;  (2)  the  San  Francisco-Marin  block,  dissected  super- 
ficially at  the  Golden  Gate  and  culminating  south  of  it  in  San 
Bruno  Mountain  and  north  of  it  in  Mount  Tamalpais;  and 


116 

(3)  the  Berkeley  Hills  block,  overlooking  the  bay  from  the 
east.  The  outlines  of  the  form  and  structure  of  these  three 
earth  blocks  are  sho^Yn  in  figure  3. 

MONTARA  BLOCK. 
FAULTS. 

The  southwest  and  south  sides  of  Montara  Mountain  form  a 
bold,  precipitous  slope,  which  is  incisively  scarred  by  steep 
ravines.  The  main  part  of  this  slope  is  on  the  quartz  diorite 
("Montara  granite"),  but  at  the  base  of  the  slope  south  of  the 
San  Mateo  quadrangle  lie  sedimentary  beds  that  are  tilted  at 
high  angles.  These  beds  are  coarse  sandstones,  which  rest 
upon  the  quartz  diorite  and  lie  directly  beneath  a  thick  body 
of  typical  bituminous  shales  of  the  Monterey  group.  It  was 
once  suggested  that  these  sandstones  might  be  Tejon  in  age,*^ 
but  it  now  seems  more  likely  that  they  are  the  basal  beds  of 
the  Monterey.  The  attitude  of  these  beds,  which  are  thus 
steeply  inclined  against  the  massive  plutonic  rock,  suggests  that 
a  fault  plane  lies  at  the  base  of  the  mountain  along  which  the 
strata  on  the  north  side  have  been  raised  and  those  on  the 
south  side  have  been  dropped,  and  this  is  doubtless  the  move- 
ment that  gave  the  northern  mass  its  asymmetric  profile — a 
profile  characteristic  of  slightly  degraded  tilted  blocks.  The 
beds  of  the  Monterey  group  that  now  lie  on  the  lower  soutliern 
flank  of  tlie  mountain  are  remnants  of  deposits  which  at  one 
time  extended  well  over  the  area  of  the  mountain  but  which 
have  been  eroded  away  on  its  uplifted  side.  A  large  part  of 
this  erosion,  however,  occurred  before  the  movement  that  gave 
the  block  its  present  profile.  From  the  crest  of  Montara 
Mountain  northward  the  general  profile  of  the  tilted  block 
descends  rather  gently  and  by  steps  to  the  Bay  of  San  Fian- 
cisco,  or  to  the  base  of  the  southwest  front  of  San  Bruno 
Mountain,  which  is  the  degraded  scarp  of  the  San  Bruno  fault. 
The  entire  Merced  formation,  which  occupies  the  northeastern 
part  of  the  Montara  block,  abuts  upon  this  fault  plane,  as 

"Jjawson,  A.  C,  Sketch  of  the  geology  of  the  San  Francisco  Peninsula: 
U.  S.  Geol.  Survey  Fifteenth  Ann.  Rept.,  p.  458,  1895. 


117 


shown  in  sections  A-A  and  B-B,  and  the  amount  of  differ- 
ential displacement  that  has  occurred  on  it  is  therefore  estimated 
at  not  less  than  7000  feet. 

The  San  Bruno  fault  is  paralleled  by  another,  which  lies 
closer  to  the  face  of  the  mountain  and  is  regarded  as  auxiliary 
to  the  main  fault,  probably  converging  downward  toward  it 
and  joining  it  far  below  the  surface.  The  trace  of  this  fault 
where  it  crosses  the  buttress-like  shoulders  of  San  Bruno 
Mountain  is  marked  by  fault  breccia  in  the  saddles. 

The  chief  structural  feature  within  the  Montara  block  is  the 
San  Andreas  fault,  which  finds  expression  at  the  surface  in 
the  well-defined  San  Andreas  rift  valley,  show^n  in  Plate  IX. 
This  is  a  long,  straight,  narrow  valley,  which  trends  about 
N.  34°  W.  In  this  rift  valley  lies  the  trace  of  the  San 
Andreas  fault,  on  w^hich  repeated  movements  have  taken  place, 
the  last  notable  movement  occurring  on  A]iril  18,  1906,  when 
there  wns  a  differential  displacement  of  such  a  nature  that  the 
country  northeast  of  the  fault  moved  horizontally  southeast- 
ward and  the  country  southwest  of  the  fault  moved  horizontally 
northwestward,  the  maximum  displacement  in  this  region  hav- 
ing been  12  to  16  feet.  In  its  prolongation  northwestward 
beneath  the  sea,  outside  of  tlie  Golden  Gate,  this  fault  con- 
verges upon  the  San  Bruno  fault,  and  the  two  appear  to  be 
coincident  in  the  rift  valley  between  Bolinas  Lagoon  and 
Tomales  Bay.  The  rift  produced  during  the  movement  in 
1906  along  this  fault  is  shown  in  Plate  X. 

The  fault  that  parallels  the  San  Andreas  fault  on  the  west 
slope  of  the  Cahil  Ridge,  in  the  Santa  Cruz  quadrangle,  prob- 
ably follows  Pilarcitos  Canyon,  in  the  San  Mateo  quadrangle, 
and  passes  through  the  length  of  the  southwest  arm  of  Pilar- 
citos Lake.  It  is  traceable  over  Whiting  Ridge  to  the  head  of 
San  Pedro  Valley,  beyond  which,  to  the  coast,  it  is  concealed 
by  the  alluvium  of  the  valley  bottom.  From  a  point  neai-  the 
southern  border  of  the  San  Mateo  quadrangle  to  a  point  within 
a  mile  of  Pilarcitos  Lake  this  fault  forms  the  boundary 
between  the  Franciscan  and  the  quartz  diorite  of  Montara 
Mountain,  and  from  the  point  last  named  to  the  coast  it  forms 
the  boundary  between  the  Franciscan  and   the  wedge-shaped 


118 

area  of  Eocene  beds  that  flank  the  north  side  of  the  mountain. 
The  movement  here  was  probably  a  thrust,  by  which  the  rock 
mass  northeast  of  the  fault  overrode  the  rock  mass  southwest 
of  it,  thereby  cutting  away  the  northeast  limb  of  the  syncline 
of  Eocene  rocks  in  the  vicinit}^  of  Pilarcitos  Lake. 

Another  fault  appears  to  follow  the  canyon  that  lies  between 
Sawyer  Ridge  on  one  side  and  Cahil  and  Fifield  ridges  on 
the  other.  There  are  also  other  minor  faults  in  the  Montara 
block,  but  their  structural  relations  have  not  been  clearly 
deciphered. 

Along  some  of  the  fault  planes  in  the  region  about  San 
Francisco  Bay  earth  movements  still  occur  and  are  accom- 
panied by  earthquakes.  The  importance  of  giving  due  con- 
sideration to  these  faults  in  connection  with  engineering 
projects  is  considered  under  the  heading  "Economic  geology." 

FOLDS. 

In  the  Franciscan  group  of  the  Montara  block  the  folding- 
is  obscure  and  diflicult  to  decipher  in  detail,  partly  because  the 
prevailing  sandstone  has  but  feebly  marked  stratification  and 
is  everywhere  very  much  the  same  in  appearance  in  its 
weathered  outcrops  and  partly  because  the  strata  include 
irregular  sheets  of  basic  igneous  rock,  some  contempora- 
neous and  some  intrusive,  and  are  also  much  broken  and  dis- 
located in  a  minor  way.  In  general,  however,  the  folds  of 
the  Franciscan  rocks  are  notably  open,  the  strata  dipping  at 
low  angles. 

Sawyer  Ridge  appears  to  be  a  very  flat  syncline  pitching 
southeastward.  (See  section  E-E.)  Cahil  Ridge  is  a  slightly 
more  appressed  syncline,  with  no  perceptible  pitch,  but  near 
its  south  end,  almost  at  the  southern  border  of  the  quadrangle, 
it  has  a  rather  complicated  twist.  This  synchne,  with  its  sub- 
ordinate folds,  probably  extends  through  to  the  coast  at  Calera 
Valley.  The  sandstone  of  Sweeny  Ridge  is  probably  anti- 
clinally  related  to  the  basalts  that  flank  it  on  both  sides. 

Tn  tlie  area  just  northeast  of  the  San  Andreas  rift  valley, 
near  Belmont,  Sausalito  chert  lies  on  the  Cahil  sandstone  in  a 


119 


flat  but  broken  syncline,  which,  on  Belmont  Hill,  locally  takes 
the  form  of  a  sharply  overturned  fold.     (See  section  G-G.) 

The  structure  of  the  sedimentary  rocks  of  Buriburi  and 
Pulgas  ridges  is  masked  by  abundant  igneous  rocks,  but  where 
the  stratification  is  exposed  the  beds  in  general  appear  to 
stand  much  steeper  than  in  the  area  southwest  of  the  San 
Andreas  rift.  A  marked  structural  feature  of  these  two  ridges 
is  their  slightly  inclined,  gently  undulating  surfaces,  which 
were  determined  by  a  laccolithic  sheet  of  peridotite,  now  ser- 
pentinized.  Associated  with  this  laccolithic  sheet  are  dikes  of 
the  same  material. 

A  notable  fold  of  the  .INIontara  block  is  that  in  wdiicli  lies 
the  tapering  belt  of  Eocene  rocks  betw^een  the  Franciscan 
rocks  and  the  quartz  diorite  of  Montara  Mountain,  extending 
from  San  Pedro  Point  beyond  Pilarcitos  Lake.  These  beds 
were  evidently  laid  down  across  the  line  of  contact  of  the 
Franciscan  and  the  quartz  diorite  and  have  since  been  buckled 
up  in  an  irregular  syncline  by  movements  antedating  the  for- 
mation of  the  Pilarcitos  fault,  an  overthrust  which  has  cut 
off  the  northeast  limb  of  the  syncline  at  the  southeast  end 
of  the  belt,  as  shown  in  section  D-D.  The  rocks  in  this 
general  trough  show  a  double  syncline  on  the  coast  section  and 
are  traversed  by  a  great  many  small  faults.  It  is  evident  that 
these  strata  yielded  much  more  readily  under  compressive 
stress  than  the  stronger  rocks  on  either  side  of  the  trough. 

Another  notable  fold  within  the  Montara  block  is  a  syncline 
on  its  northeast  margin,  which  involves  the  Merced  formation. 
The  southwest  limb  of  this  syncline  is  well  exposed  in  the 
sea  cliffs  between  Merced  Lake  and  Mussel  Rock,  where  the 
Merced  strata  dip  uniformly  to  the  southeast  at  angles  ranging 
from  15°  to  75°  but  are  broken  by  numerous  small  normal 
faults,  which  have  throws  ranging  from  a  few  inches  to  a  few" 
feet.  The  strata  are  over  a  mile  thick  in  the  measured  section 
of  the  sea  cliffs,  and  the  dip  of  this  great  volume  of  sedi- 
mentary beds  indicates  that  they  abut  upon  tlie  San  Bruno 
fault  beneath  Merced  Valley,  as  shown  in  sections  A-A  and 
B-B.  If  the  Merced  strata  were  folded  before  the  San  Bruno 
fault  was  formed,  as  seems  probable,  then  the  nortlieast  Hmb 


120 


of  the  fold  lias  been  carried  up  by  the  fault  and  removed  by 
erosion. 

The  Merced  strata  are  similarly  preserved  in  the  same 
syncline  in  the  rift  valley  northwest  of  Bolinas  Lagoon,  in  the 
Tamalpais  quadrangle.  (See  section  A-A.)  The  underlying 
bituminous  shales  of  the  Monterey  group  that  occupy  the 
south  end  of  the  Point  Keyes  Peninsula  are  much  broken 
and  confused  in  structure,  but  the  outcrops  of  these  shales  on 
the  shore  between  Bolinas  and  Duxbury  Point  display  part  of 
an  anticline  in  which  the  strata  dip  prevailingly  toward  the 
northwest.  The  relation  of  these  beds  to  the  San  Bruno  and 
San  Andreas  faults  is  further  described  under  the  heading 
"San  Francisco-Marin  block." 

Some  interesting  folding  of  Merced  strata  occurs  at  Mira- 
montes  Ridge  and  on  the  wave-cut  terrace  at  its  base,  in  the 
southwest  corner  of  the  San  Mateo  quadrangle.  This  area  of 
Merced  lies  beyond  the  southwestern  limit  of  the  Montara 
block,  but  its  structural  features  may  conveniently  be  men- 
tioned here.  The  Merced  formation  rests  directly  upon  the 
surface  of  the  quartz  diorite,  its  basal  deposits  being  those  of 
a  bowlder  and  pebble  beach  and  containing  abundant  n)arine 
fossils  of  beach  habitat.  These  beach  deposits  are  firmly 
cemented,  but  the  beds  immediately  above  them  show  notable 
differences  in  resistance  to  marine  corrasion,  some  being  easily 
worn  away  and  others  remaining  as  salient  reefs.  The  strata 
are  folded  into  a  series  of  small  synclines  and  anticlines,  and 
in  the  development  of  the  wave-cut  terrace  the  arches  or  domes 
of  many  of  these  small  anticlines  have  been  truncated.  The 
hard  beds  of  these  truncated  anticlines  form  circular  or  oval 
reefs  that  inclose  depressions  out  of  which  the  softer  under- 
lying beds  have  been  washed  by  the  w^aves.  Between  these 
circular  reefs  lie  the  sinuous  depressions  of  the  intervening 
synclines,  from  whicli  soft  beds  that  lay  above  the  hard  reefs 
have  been  similarly  scoured  away.  At  low  tide  there  is  thus 
presented  to  view  over  the  broad  terrace  a  remarkable  model 
of  the  folded  structure.  The  structure  is,  however,  even  more 
remarkable  than  it  apy^ears  superficially,  for  the  folded  strata 
rest  upon  an  early  Merced,  relatively  smooth  wave-cut  surface 


121 

of  the  quartz  diorite,  as  shown  in  sections  C-C  and  D-D,  and 
it  is  difficult  to  picture  in  the  mind  the  adjustments  that  have 
taken  phice  within  these  Merced  beds.  It  seems  ahnost  neces- 
sary to  assume  that  they  had  been  crowded  horizontally  over 
the  flat  quartz  diorite  surface  upon  which  they  rest,  and  that 
the  forces  that  thus  folded  them  had  not  greatly  deformed  the 
underlying  massive  rock. 

SAN  FRANCISCO-MARIN  BLOCK. 
FAULTS. 

That  part  of  the  San  Francisco-Marin  block  which  lies  in 
the  peninsula  south  of  the  Golden  Gate  is  bounded  on  the 
southwest  by  the  San  Bruno  fjuilt.  The  general  features  of 
the  degraded  San  Bruno  scarp  are  continued  in  the  Marin 
Peninsula  by  the  steep  slope  that  overlooks  the  Pacific  and 
the  rift  valley  between  Bolinas  Lagoon  and  Tomales  Bay.  It 
is  therefore  probable  that  the  San  Bruno  fault  extends  across 
the  sea  floor  outside  the  Golden  Gate  and  follows  the  rift  val- 
ley from  Bolinas  Lagoon  northw^estward.  (See  fig.  3.)  This 
interpretation  is  supported  by  the  fact  that  at  Bolinas  the 
Merced  strata  dip  northeastward  and  appear  to  abut  upon  the 
feult,  as  shown  in  section  A-A,  just  as  they  do  on  the  San 
Francisco  Peninsula.  But  on  entering  the  rift  valley  the 
San  Bruno  ftiult  becomes  coincident  with  the  zone  of  the  San 
Andreas  fault,  which  is  probably  a  later  feature  of  the  struc- 
ture of  the  region  and  w^hich  locally  followed  the  line  of 
weakness  and  rupture  already  established  by  the  San  Bruno 
fault. 

The  history  of  displacement  along  the  zone  of  faulting  in 
the  rift  valley  of  Marin  County  is  long  and  complicated.  The 
rocks  on  the  two  sides  of  the  fault  zone  are  very  different  and 
owe  their  juxtaposition  to  the  faulting.  On  the  northeast  side 
lies  a  great  thickness  of  Franciscan  strata,  with  which  are 
associated  igneous  rocks.  On  the  southwest  side  there  are  no 
Franciscan  rocks,  but  in  the  area  northwest  of  the  Tamalpais 
quadrangle  there  is  an  extensive  body  of  pre-Franciscan  granitic 


122 


rock,  which  is  overlain  by  strata  of  the  Monterey  group,"  and 
these  are  in  turn  unconforraably  overlain  by  Merced  strata.  It 
would  therefore  seem  probable  that  the  earlier  movements  on 
this  fault  zone  were  pre-Miocene  and  that  they  caused  a  rela- 
tive upthrow  on  the  southwest  side  of  the  fault,  in  consequence 
of  which  the  Franciscan  rocks  were  lifted  into  the  zone  of 
erosion  and  stripped  off  the  underlying  granitic  rock.  This 
erosion  may  have  taken  place  in  any  part  or  during  the  whole 
of  Cretaceous  and  Eocene  time. 

In  the  southern  part  of  the  Point  Reyes  Peninsula  there  is 
a  great  thickness  of  bituminous  shale  of  the  Monterey  group. 
The  shore  line  of  the  sea  in  which  these  bituminous  shales 
were  deposited  must  have  lain  far  east  of  Bolinas  Pidge,  for  we 
can  not  regard  the  beds  at  the  western  base  of  the  ridge  as  in 
any  sense  littoral.  It  follows  that  the  Monterey  beds  were 
laid  doAvn  not  only  over  the  area  of  the  Point  Reyes  Peninsula 
but  also  over  a  large  part  of  the  territory  farther  northeast, 
and  that  they  were  therefore  spread  over  the  trace  of  the  old 
fault.  In  post-Miocene  time  there  was  probably  a  recurrence 
of  movement  on  the  fault  plane  at  the  time  of  the  deformation 
of  the  Monterey  strata  and  their  uplift  into  the  zone  of  erosion, 
but  the  effect  of  this  movement  can  not  be  satisfactorily  differ- 
entiated from  that  of  a  later  post-Pliocene  displacement. 
After  the  erosion  of  part  of  the  folded  and  crushed  Monterey 
strata  the  region  was  again  depressed  and  received  the  Merced 
deposits,  and  at  the  close  of  the  Pliocene  epoch  there  was  a 
great  displacement  on  the  San  Bruno  fault.  It  was  this  move- 
ment which  raised  the  southwest  margin  of  the  San  Francisco- 
Marin  block  and  depressed  the  northeast  margin  of  the 
Montara  block.  In  consequence  of  this  uplift  the  Merced 
strata  on  the  northeast  side  of  the  fault  zone  were  completely 
removed  by  later  erosion,  and  whatever  Monterey  strata 
remained  over  that  region  after  the  post-Monterey  period  of 
erosion  were  also  removed.  The  facts  thus  stated  and  partic- 
ularly the  similar  relation  of  the  Merced  strata  to  the  San 
Bruno  fault  on  l)oth  sides  of  the  Golden  Gate  show  that  the 


"Anderson,  F.  M.,  The  geology  of  the  Point  Reyes  Peninsula:  California 

Univ.  Dept.  (ieology  Bull.,  vol.  2,  No.  5,  1899. 


Point  Reyes  Peninsula  is  orogenically  a  part  of  the  Montara 
block. 

In  later  Quaternary  time,  subsequent  to  the  large  displace- 
ments that  are  represented  by  the  San  Bruno  fault,  there 
began  the  movements  which  find  expression  in  the  San 
Andreas  fault — movements  which  are  still  in  progress  but  are 
as  yet  relatively  small  and  are  characterized  by  a  great  excess 
of  their  horizontal  over  their  vertical  component.  In  the 
Tamalpais  quadrangle  the  trace  of  the  San  Andreas  fault  is 
coincident  with  that  of  the  San  Bruno  fault,  but  in  the  area 
south  of  the  Golden  Gate  the  line  of  the  San  Andreas  fault  is 
separate  and  divergent  from  the  older  line  of  dislocation. 

Within  the  San  Francisco-Marin  block  there  are  many 
minor  faults,  some  of  which  are  indicated  on  the  geologic 
maps.  One  of  these  is  well  exposed  on  the  sea  cliffs  about 
three-quarters  of  a  mile  south  of  Fort  Point,  San  Francisco. 
By  this  fault  a  band  of  radiolarian  chert  of  the  Franciscan 
group  (probably  Ingleside)  is  brought  against  sandstone  of  the 
same  group  (probably  Marin).  The  fault  doubtless  extends 
southeastward  across  the  city  of  San  Francisco,  but  back  from 
the  shore  its  trace  is  obscure.  A  number  of  faults  in  the 
southern  part  of  the  Marin  Peninsula  have  been  recognized 
by  dislocations  which  they  cause  in  the  Franciscan  strata.  In 
the  main  portion  of  the  Franciscan  group,  however,  where 
the  rocks  are  prevailingly  sandstone,  similar  faults  are  dif- 
ficult to  detect,  and  even  if  a  fault  is  observed  at  one  place 
it  is  difficult  to  follow  and  map.  The  region  therefore  prob- 
ably contains  many  more  faults  than  are  indicated  on  the 
geologic  maps. 

FOLDS. 

In  general,  the  folds  of  the  Franciscan  rocks  in  the  San 
Francisco-Marin  block  show  remarkably  little  appression,  par- 
ticularly in  their  larger  features,  but  in  parts  of  the  Franciscan 
terrane  where  the  rocks  are  thin  bedded  and  therefore  incom-, 
petent  to  transmit  pressure  they  show  the  results  of  great 
crushing,  the  beds  having  been  broken  and  crumbled.  The 
strike  of  the  folded  beds  is  in  general  inconstant,  even  where 


124 


the  strata  are  highly  inclined,  and  the  axes  of  such  folds 
as  can  be  made  out  are  also  very  diverse  in  direction.  This 
irregularity  of  structure  is  doubtless  due  to  the  fact  that  the 
folding  is  the  result  of  a  succession  of  earth  movements  having 
different  directions.  The  irregular  folding  in  the  Franciscan 
strata  presents  a  striking  contrast  to  the  folding  in  the  later 
sedimentary  beds  of  the  Coast  Ranges,  which  is  simpler  and 
more  like  the  Appalachian  type. 

The  prevailing  dip  of  the  Franciscan  rocks  on  the  San  Fran- 
cisco Peninsula  is  to  the  northeast.  The  San  Miguel  Hills, 
on  the  southwest  edge  of  the  city,  lie  in  an  undulating  syn- 
cline,  the  sandstone  of  the  southwest  flank  of  the  hills  passing 
under  the  cherts  of  the  summits  at  a  low  angle.  The  sand- 
stones at  the  northwest  end  of  San  Bruno  Mountain,  near 
Ocean  View,  dip  to  the  southwest,  their  attitude  indicating 
that  they  lie  on  the  southwest  limb  of  an  anticline,  but  the 
strata  here  differ  from  the  the  usual  sandstones  of  the  Fran- 
ciscan and  may  possibly  belong  to  the  Cretaceous  system. 
The  structure  northeast  of  the  San  Miguel  syncline  is  obscured 
by  bodies  of  serpentine  and  by  an  extensive  mantle  of  dune 
sand.  The  sandstones  south  of  Fort  Point  are  very  evenly 
stratified  and  dip  to  the  northeast,  and  as  those  on  the  south 
side  of  Hunter  Point,  although  intensely  contorted,  appear  to 
have  the  same  general  dip,  their  attitude  suggests  that  they  lie 
in  the  northern  limb  of  an  anticline.  On  Pincon  Hill,  above 
Rincon  Point,  the  prevailing  dip  is  similarly  to  the  northeast. 
In  the  San  Miguel  Hills  and  in  the  neighboring  liills  the 
sequence  of  the  Franciscan  formations  and  the  character  of 
the  folding  can  be  made  out  fairly  well,  but  the  structure 
elsewhere  in  the  city  of  San  Francisco  is  obscure.  In  many 
exposures  in  street  cuts  and  other  excavations  even  the  local 
dip  can  not  be  determined,  owing  to  the  mashing  of  the  softer 
strata,  the  complications  due  to  intrusive  rocks,  the  depth  of 
rock  decomposition,  and  the  surftice  creep.  The  general  struc- 
ture across  tlie  southeastern  part  of  the  city,  however,  is  shown 
in  section  E-E. 

On  tlie  Marin  and  Tiburon  peninsulas  certain  broad  features 
of  the  folding  can  be  made  out,  but  a  detailed  interpretation  of 


125 


tlie  structure  is  possible  only  locally.  On  the  northeast  sides 
of  Tihuron  Peninsula  and  Angel  Island  the  dips  are  southwest, 
but  at  Belvedere  and  on  the  southwest  side  of  Angel  Island 
the  dips  are  northeast.  The  structure  of  Tiburon  Peninsula 
and  Angel  Island  is  therefore  synclinal,  though  complicated  by 
numerous  intrusions.  (See  sections  B-B  and  C-C.)  On  the 
hills  northwest  of  Sausalito  the  general  dip  is  to  the  southwest 
or  south,  so  that  Richardson  Bay  and  Strawberry  Point  lie  in 
an  anticline.  At  the  south  end  of  the  Marin  Peninsula,  west 
of  Sausalito,  the  strike  swings  from  northwest  to  west  and  the 
dip  from  southwest  to  south.  The  general  structure  is  that 
of  an  open  syneline  of  low  pitch  to  the  southwest.  It  is 
noteworthy  that  much  of  the  stratification  at  this  end  of  the 
peninsula  is  transverse  to  the  San  Bruno  fault  and  probably 
abuts  against  it  farther  west,  under  the  ocean. 

From  Frank  Valley  northwestward  to  the  northwest  corner 
of  the  Tamalpais  quadrangle  the  strike  becomes  parallel  to  the 
trend  of  Bolinas  Ridge,  and  the  prevailing  dip  from  the  base 
of  the  ridge  to  its  summit  is  northeastward  at  moderate  angles. 
This  would  indicate  that  Mount  Tamalpais  occupies  the  axis 
of  a  broad  syneline  (see  section  A-A),  perhaps  the  same  s\^n- 
cline  that  is  recognized  on  the  Tiburon  Peninsula.  The  strata 
north  and  northeast  of  San  Rafael  dip  northward  and  the 
ridge  that  terminates  in  Point  San  Quentin  would  therefore 
appear  to  be  an  anticline  lying  between  the  Tiburon  Peninsula 
syneline  and  another  synclinal  fold  on  the  northern  border  of 
the  Tamalpais  quadrangle.  In  figure  3  these  are  named  the 
San  Quentin  anticline  and  San  Rafael  syneline.  The  prevail- 
ing dip  on  the  Potrero  San  Pablo  Peninsula  is  southwestward, 
though  this  dip  is  at  several  places  locally  reversed.  The 
sandstones  on  Goat  Island  dip  northeastward  but  show  local 
small  folds,  some  of  which  are  broken  and  overthrust. 

BERKELEY  HILLS  BLOCK. 
FAULTS. 

The  Berkele}''  Hills  orogenic  block  is  bounded  on  its  south- 
west or  uplifted  margin  by  a  zone  of  acute  deformation,  which 
extends  through  the  San  Francisco,  Concord,  and   Havwards 


126 


quadrangles.  At  several  places  in  this  zone  a  fault,  named  the 
Haywards  fauh,  is  manifested  by  geomorphic  features  similar 
to  those  of  the  Ban  Andreas  rift,  and  in  the  same  zone  there 
are  numerous  subordinate  faults,  many  of  them  more  or  less 
oblique  to  the  general  trend  of  the  rift,  and  some  of  them  even 
transverse  to  it. 

Notable  manifestations  of  rift  geomorphy  occur  in  all  three 
quadrangles,  but  the  faults  that  caused  them  are  not  mapped 
because  it  has  not  been  possible  to  determine  their  locations 
exactly.  Perhaps  the  most  striking  manifestation  is  that  in 
the  Concord  quadrangle,  between  Claremont  Creek  and  the 
Arroyo  Viejo.  Along  the  foothills  between  these  points  lies 
a  long,  narrow  interrupted  valley  which  is  parallel  to  the  trend 
of  the  range.  The  drainage  lines  of  the  w^est  slope  of  the 
Berkeley  Hills  are  consequent  upon  the  surface  resulting 
from  the  uplift  of  this  margin  of  the  block  and  run  down 
the  steep  slope  transverse  to  the  trend  of  the  range.  This 
narrow  longitudinal  valley  is  therefore  a  strikingly  excep- 
tional feature  of  the  slope.  Most  of  the  transverse  conse- 
quent streams  whose  valleys  intersect  the  longitudinal  valley 
in  passing  down  the  slope  are  diverted  for  short  distances 
along  the  longitudinal  valley  before  they  pass  southwest- 
ward  through  breaches  in  its  wall.  (See  fig.  4.)  Kohler 
Creek  and  Hayes  Creek  appear  to  represent  an  originally 
continuous  consequent  drainageway  that  has  been  divided  into 
two  parts  by  the  longitudinal  valley,  along  which  the  upper 
stretch  of  the  stream  has  been  diverted.  The  Arroyo  Viejo, 
similarly,  may  once  have  flowed  through  a  notch  in  the  ridge 
which  now  bars  its  path  to  the  southwest  but  was  deflected 
to  the  north  along  the  longitudinal  valley.  A  typical  trans- 
verse consequent  stream,  however,  Shepard  Creek  and  its 
continuation  as  Diamond  Creek,  shows  very  little  deflection, 
and  the  «;ori'e  in  which  it  flows  is  continuous  on  both  sides 
of  the  longitudinal  valley. 

It  would  thus  appear  that  the  consequent  drainage  of  the 
southwest  slope  of  the  Berkeley  Hills,  though  still  immature, 
nevertheless  antedates  the  longitudinal  valley,  which  is  prob- 
ably in  part  diastrophic  in  origin  and  was  in  part  formed  by 


127 


the  more  rapid  erosion  of  subsequent  drainage  along  a  line  of 
exceptional  weakness  due  to  the  faulting.  Therefore,  as  the 
consequent  drainage  was  begun  by  the  uptilt  of  the  Berkeley 


Scale 

2 


3  Miles 


Figure  4.— Outline  map  of  the  western  slope  of  the  Berkeley  Hills  in  the 
southwestern  part  of  the  Concord  quadrangle  and  adjacent  parts  of 
the  San  Francisco  and  Haywards  quadrangles,  showing  the  deflection 
of  streams  by  the  longitudinal  rift  valley  of  the  Haywards  fault  zone. 

Several  of  the  southwestward-flowing  cousequent  streams  follow  the  longitudinal  valley  for  a 
short  distance  before  continuing  their  southwestward  courses. 

Hills  block,  it  follows  that  the  Haywards  fault,  which  finds  its 
chief  expression  in  the  longitudinal  rift  valley,  occurred  subse- 
quent to  the  development  of  the  main  fault  zone  between  the 


128 


Berkeley  Hills  block  and  the  San  Francisco-jNIarin  block.  In 
this  respect  the  relation  of  the  Havwards  fault  to  this  fault 
zone  is  analogous  to  that  of  the  San  Andreas  fault  to  the  earlier 
San  Bruno  fault.  The  Hay  wards  fault  may  therefore  mark  the 
advent  of  an  earth  movement  that  was  unrelated  to  and  differ- 
ent from  that  which  displaced  the  large  blocks,  just  as  the  San 
Andreas  fault  appears  to  have  been  unrelated  to  the  movement 
that  displaced  the  Montara  and  the  San  Francisco-Marin 
blocks,  although  it  is  in  part  of  its  course  coincident  with  the 
San  Bruno  fault. 

The  rift  valley  which  is  so  well  developed  in  the  Concord 
quadrangle  extends  only  a  short  distance  southward,  into  the 
Haywards  quadrangle,  as  a  continuation  of  the  northwest- 
southeast  depression  in  which  a  part  of  the  Arroyo  Viejo  flows. 
Beyond  this  point  toward  Haywards,  however,  the  trace  of  the 
Haywards  fault  may  be  followed  southeastward  through  a  line 
of  sags  and  saddles  in  'the  hills  west  of  Lake  Chabot,  until,  at 
Haywards,  it  passes  out  to  the  base  of  the  hills.  The  faults 
mapped  between  Haywards  and  the  edge  of  the  quadrangle, 
east  of  Decoto,  and  those  shown  in  the  vicinity  of  Lake  Chabot 
are  features  of  the  general  zone  of  deformation  that  bounds  the 
Berkeley  Hills  on  the  southwest  and  probably  represent  the 
earlier  movement,  which  defined  the  Berkeley  Hills  block, 
rather  than  that  which  produced  the  Haywards  fault. 

The  zone  of  deformation  on  the  southwest  flank  of  the 
Berkeley  Hills  is  marked  in  the  San  Francisco  quadrangle  by 
a  series  of  step  faults,  the  effects  of  which  are  still  clearly 
defined  in  the  profile  of  the  slope  between  Strawberry  Creek 
and  Cordonices  Creek.  Beyond  Cordonices  Creek  similar 
faults  are  indicated  by  stratigraphic  displacements,  although 
they  are  only  feebly  expressed  in  the  geomorphic  profile.  In 
the  district  known  as  Cragmont,  east  of  Northbrae,  one  of  the 
fiults  is  marked  bv  abundant  coarse  fault  breccia,  in  a  silicified 
facies  of  the  soda  rhyolite  which  outcrops  as  a  bold  knob  on 
the  slope.  On  the  crest  of  the  northwest  end  of  the  ridge  that 
separates  Wildcat  Canyon  from  the  valley  of  the  Bay  of  San 
Francisco  there  is  a  saddle-like  depression,  which  lies  parallel 
to  the  crest,  ;ind  in  the  line  of  this  depression  there  are  several 


129 


sinks  or  undrained  ponds,  features  that  have  their  counterpart 
in  certain  places  along  the  San  Andreas  rift  and  are  believed 
to  have  had  a  similar  seismic  origin. 

The  more  notable  transverse  faults  that  cross  the  zone  of 
deformation  are  at  Cordonices  Creek,  Strawberry  Creek, 
Hamilton  Gulch,  Temescal  Creek,  and  San  Leandro  Creek. 

Of  the  numerous  other  faults  in  the  Berkeley  Mills  block 
only  the  more  important  will  be  mentioned  here.  They  are 
shown  in  figure  3.  A  large  and  well-defined  displacement 
on  a  nearly  vertical  fault  plane  having  a  general  northwest 
strike  traverses  Strawberry  and  Claremont  canyons.  The 
downthrow  is  on  the  southwest  side  and  amounts  to  several 
hundred  feet.  This  fault  is  traceable  southeastward  beyond 
the  head  of  Temescal  Canyon.  In  Strawberry  Canyon  it 
intei'sects  tw^o  of  the  transverse  faults,  and  in  the  triangle 
between  them  a  block  of  soft  Quaternary  beds  (Campus  for- 
mation) has  been  dropped  against  the  Cretaceous  rocks.  The 
fault  is  traceable  northwestward  beyond  Strawberry  Canyon 
for  several  miles  along  the  northeast  side  of  Wildcat  Can- 
yon and  may  therefore  be  called  the  Wildcat  fault.  In  one 
part  of  the  canyon,  on  its  southwest  side,  the  Orinda  strata, 
dipping  to  the  northeast,  appear  to  abut  upon  the  Franciscan 
rocks,  exposed  on  the  northeast  side,  but  because  of  the  small- 
ness  of  the  exposure  of  the  Franciscan  beds  they  are  not  so 
mapped. 

A  notable  fault  enters  the  San  Francisco  quadrangle  from 
the  north  in  the  valley  of  Pinole  Creek.  The  downthrow  is 
on  the  west  side,  which  brings  the  tuff  of  the  basal  part  of  the 
Orinda  formation  against  several  diiTerent  divisions  of  the 
Monterey.  This  fault  may  be  referred  to  as  the  Pinole  fault. 
About  a  mile  and  a  half  south  of  the  northern  limit  of  the 
quadrangle  the  line  of  faulting  leaves  Pinole  Creek  and,  pass- 
ing through  the  hills  on  its  south  side,  enters  the  Concord 
quadrangle  near  the  summit  of  Sobrante  Ridge,  which  it  fol- 
lows as  far  as  Bear  Creek.  Here  it  turns  southward  and 
changes  from  a  longitudinal  or  strike  fault  to  a  transverse 
fjuilt,  cutting  across  the  trend  of  the  folded  strata.  In  the 
southern   part  of  its  course  the  fault   may  be  followed  down 

San  rraufisco~9 


130 


Bear  Creek,  across  San  Pablo  Creek,  and  thence  to  the  crest  of 
San  Pablo  Ridge  and  into  Siesta  Valley,  where  it  appears  to 
die  out.  This  fault  traverses  strata  that  had  been  previously 
folded,  and  the  discordance  due  to  displacement  is  very  strik- 
ingly shown  in  many  sections  along  its  course,  the  downthrow 
being  uniformly  on  the  southwest  or  west  side.  It  is  appar- 
ently due  to  overthrust  from  the  northeast  (see  section  F-F  on 
the  San  Francisco  map),  and  at  its  maximum  displacement 
it  brings  the  Tejon  formation  against  the  upper  part  of  the 
Monterey. 

A  less  persistent  fault  enters  the  same  group  of  hills  from 
San  Pablo  V^alle}''  at  a  point  about  3  miles  east  of  the  town  of 
San  Pablo  and,  with  a  southeast  strike,  conv^erges  toward  the 
Pinole  fault  for  a  distance  of  about  3  miles.  Its  strike  is 
parallel  to  that  of  the  strata,  but  its  structural  effect  is  not 
altogether  clear. 

Another  remarkabl}^  curved  fault  line  appears  at  the  south- 
east end  of  the  Bear  Creek  anticline,  in  the  northwestern  part 
of  the  Concord  quadrangle.  This  fold  is  broken  and  over- 
thrust  bv  a  fault  havino-  a  northwest-southeast  strike.  The 
fault  follows  Briones  Valley  for  2^  miles  from  Bear  Creek  and 
then  turns  sharply  to  the  north,  across  the  Briones  Hills,  and 
crosses  Vaca  Canyon  to  a  shoulder  of  Franklin  Ridge,  where 
it  is  cut  off  by  another  fault,  which  extends  along  the  south- 
west side  of  Franklin  Ridge  and  which  will  be  referred  to 
later.  This  curving  fault,  which  may  be  called  the  Briones 
fault,  also  cuts  strata  that  had  been  previously  folded,  and  the 
apparent  horizontal  displacement  of  the  faulted  formations  is 
very  marked  and  easily  mapped.  The  curvature  of  the  fault 
trace  and  the  character  of  the  displacements  of  the  surface  out- 
crops suggest  that  the  fault  movement  was  an  overthrust  on  a 
fiat-lying  plane,  involving  a  slight  rotation  of  the  overthrust 
slab  on  a  vertical  axis. 

A  prominent  fault,  here  named  the  Franklin  fault,  enters 
the  Concord  quadrangle  near  the  west  end  of  the  Franklin 
Canyon  tunnel,  on  the  Santa  Fe  Railway.  It  is  an  overthrust 
fault  by  which  the  Cretaceous  rocks  on  the  northeast  have 
been  caused  to  override  the  Monterey  rocks  on  the  southwest. 


131 


The  thrust  plane  passes  through  the  tunnel  with  a  low  dip  to 
the  northeast,  and  the  sliales  of  the  Monterey  group,  which  in 
the  tunnel  lie  l)eneath  the  Cretaceous  sandstones  tliat  outcrop 
at  the  surface,  have  been  reduced  by  the  movement  to  a  clayey 
mass  which  when  wet  flows  into  the  tunnel,  so  that  the  rail- 
way engineers  have  had  great  trouble  in  keeping  it  open. 
West  of  the  trace  of  the  fault  there  are  isolated  patches  of 
Cretaceous  strata,  which  lie  upon  the  Monterey.  Three  of 
these  patches  are  shown  near  the  top  of  the  map.  These  are 
residual  parts  of  a  former  extension  of  the  overthrust  Creta- 
ceous terrane.  For  nearly  a  mile  and  a  half  south  of  tlie 
northern  limit  of  the  Concord  quadrangle  the  trace  of  this  flat 
thrust  is  the  boundary  between  the  Cretaceous  and  the  ISIon- 
terey.  Farther  south  it  is  the  boundary  between  the  Martinez 
and  the  Monterey  for  a  little  over  3^  miles,  and  beyond  this, 
still  farther  south,  it  is  the  boundary  between  the  Tejon  and 
the  Monterey  for  about  4  miles.  At  the  end  of  this  stretch 
the  thrust  is  dislocated  by  a  transverse  fault  and  its  trace  is  offset 
over  an  eighth  of  a  mile,  the  horizontal  dis]~»lacement  being  to 
the  east  on  the  south  side  of  the  transverse  fault.  South  of  this 
point  the  fault  plane  is  distinctly  traceable  for  more  than  5h 
miles  as  the  boundary  between  the  Tejon  and  the  upper  beds  of 
the  Monterey  group  and  the  San  Pablo  formation.  Its  strike 
is  the  same  as  that  of  the  strata  of  the  overthrust  block.  For 
the  next  mile  and  a  quarter  it  is  not  so  clearly  traceable,  but  it 
probably  continues  southward  and  joins  obliquely  anotlier 
thrust  fault  on  the  northeast  flank  of  Las  Trampas  Ridge,  at  a 
point  about  a  mile  west  of  Danville,  at  the  eastern  edge  of  the 
quadrangle. 

The  thrust  fault  last  mentioned  may  be  called  the  Las 
Trampas  fault,  since  it  crosses  the  ridge  of  that  name  obliquel3\ 
It  extends  from  the  vicinity  of  Lafayette  to  a  point  about  a 
mile  south  of  Danville,  at  the  eastern  edge  of  the  Concord 
quadrangle,  beyond  wliich  it  is  lost  in  tlie  alluviated  floor  of  Ban 
Ramon  Valley.  It  brings  several  horizons  of  the  Monterey 
against  the  San  Pablo  along  the  main  portion  of  its  course, 
and  its  strike  is  in  general  parallel  to  that  of  the  strata  of  the 
overthrust  block  on  the   southwest.     The  movement  on   tliis 


132 


fault  was  from  the  southwest,  in  a  direction  opposite  to  that  of 
the  Franklin  fault,  so  that  where  the  two  faults  meet,  west 
of  Danville,  the  fault  planes  intersect  like  the  blades  of  a  pair 
of  shears,  the  plane  of  the  Las  Trampas  fault  being  the  upper 
blade  and  so  cutting  oat  the  Franklin  feult  at  the  surface,  as 
shown  in  section  E-E. 

On  the  southwest  side  of  Bolinger  Canyon,  well  up  toward 
the  crest  of  Rocky  Ridge,  there  is  another  notable  thrust  fault, 
which  may  be  called  the  Bolinger  fault.  For  a  considerable 
part  of  its  course  its  trace  is  roughly  parallel  to  that  of  the 
Las  Trampas  fault,  and  the  displacement  is  in  the  same  direc- 
tion in  the  two  thrusts — that  is,  the  rocks  southwest  of  the  fault 
line  have  overridden  those  northeast  of  it.  As  a  result  of  this 
thrust  the  Monterey  rocks  of  the  crest  of  Rocky  Ridge  overlie 
and  dip  away  from  the  Orinda  formation  on  the  southwest 
side  of  Bolinger  Canyon.  The  trace  of  this  fault  farther  north, 
beyond  the  head  of  Bolinger  Canyon,  swings  westward  down 
Las  Trampas  Creek  and  can  not  be  found  in  the  alluviated 
valleys  of  the  Orinda  formation.  It  is  probable,  however,  that 
the  thrust  movement  was  taken  up  by  the  overturned  syncline 
that  extends  northwestward  from  Moraga  Valley.  This  over- 
turned syncline  becomes  more  and  more  symmetrical  in  cross 
section  toward  the  northwest  and  finally,  east  of  Berkeley, 
becomes  a  simple  open  syncline. 

A  transverse  fault  whose  downthrow  is  on  the  southeast  side 
extends  from  the  Las  Trampas  fault  to  the  Bolinger  fault, 
passing  just  north  of  Las  Trampas  Peak.  This  transverse 
fault  appears  to  have  antedated  both  the  thrust  faults  between 
which  it  lies.  Near  the  south  end  of  Rocky  Ridge  two  paral- 
lel transverse  faults  are  cut  at  nearly  right  angles  by  the 
Bolinger  fault,  A  narrow  mass  of  rock  between  these  two 
faults  appears  to  have  dropped  in  the  manner  of  a  graben. 
Farther  south,  at  the  end  of  the  ridge,  is  another  transverse 
fault,  with  larger  throw,  the  trace  of  which  lies  in  the  line  of 
Crow  Creek.  The  displacement  of  the  strata  on  this  fault  is 
to  tlie  southwest  on  the  south  side  of  the  fault,  indicating  an 
upthrow  on  this  side. 


FOLDS. 

The  folding  of  the  strata  in  the  Berkeley  Hills  block  is 
strongly  marked  and  for  the  most  part  may  be  easily  deciphered, 
owing  to  the  contrast  between  the  different  formations  and  the 
abundant  fossils  that  many  of  them  contain.  The  general 
structure  between  the  southwest  front  of  the  Berkeley  Hills 
and  Mount  Diablo  is  that  of  a  great  synclinoriura.  The  basal 
rocks  having  this  general  structure  are  Franciscan.  They 
appear  on  the  lower  flanks  of  the  southwest  front  of  the 
Berkele}^  Hills  and  again  in  Mount  Diablo,  in  the  country 
adjoining  the  Concord  quadrangle  on  the  east.  Within  this 
general  synclinorium  are  many  folds,  some  simple  and  open, 
others  appressed  and  overturned.  The  folds  are  complicated 
by  numerous  faults,  already  described,  most  of  which  appear  to 
have  been  developed  after  the  folding  of  the  country.  The 
axes  of  the  folds  and  the  faults  are  shown  in  figure  3.  The 
most  persistent  of  the  folds  is  the  synclinal  trough  in  which 
lies  a  broad  belt  of  the  Oiinda  formation,  extending  from 
Sobrante,  in  the  San  Francisco  quadrangle,  to  the  southeast 
corner  of  the  Concord  quadrangle,  and  called  the  San  Pablo 
syncline.  (See  section  F-F,  San  Francisco  map,  and  sections 
D-D  and  E-E,  Concord  map.) 

Southwest  of  the  San  Pablo  syncline,  on  the  slopes  of  San 
Pablo  Valley  above  the  town  of  Orinda,  is  an  anticline,  which 
fades  out  rapidly  toward  the  northwest  but  which  persists  for 
several  miles  toward  the  southeast.  Southwest  of  this  anticline 
is  a  well-marked  syncline,  the  axis  of  which  extends  from  the 
head  of  Wildcat  Canyon  through  Siesta  Valley  to  Moraga 
Valley  and  possibly  beyond.  It  may  be  called  the  Siesta 
syncline.  This  fold  affects  not  only  the  Orinda  formation  but 
also  the  volcanic  and  fresh-water  formations  of  the  Berkeley 
group,  as  shown  in  section  B-B.  In  the  hills  back  of  Berkeley 
the  fold  becomes  an  open  syncline,  symmetric  in  so  far  as 
the  dips  of  the  strata  are  concerned,  but  stratigraphically 
asymmetric,  owing  to  the  lenticular  form  of  the  beds  involved. 
Southeastward  from  Bald  Peak  toward  Moraga  Valley  the 
Siesta  syncline  becomes  gradually  more  closely  appressed  and 


134 


asymmetric  and  finally  passes  into  a  strongly  overturned 
syncline  with  isoclinal  dips  toward  the  southwest  on  both 
limbs  of  the  fold.  It  is  evident  that  southeastward  from  Bald 
Peak  the  compression  becomes  more  and  more  intense,  involv- 
ing overthrust  to  the  northeast  or  underthrust  to  the  southwest. 
Still  farther  southeast  the  compression  probably  failed  to  find 
relief  in  folding  alone  but  expressed  itself  in  the  great  over- 
thrust  fault  which  has  been  called  the  Bolinger  fault.  If  this 
is  true,  then  the  overthrust  faulting  is  referable  to  the  same 
earth  movements  that  induced  the  latest  folding  of  the  region, 
though,  as  already  stated,  it  appears  generall}^  to  be  subsequent 
to  the  folding. 

Northeast  of  the  Pinole  fault  is  a  notable  anticline,  known 
as  the  Bear  Creek  anticline.  In  the  center  of  this  anticline  lie 
the  sandstones  and  shales  of  the  Tejon  formation,  which  are 
flanked  on  the  northeast  by  the  Monterey  strata,  in  normal 
sequence.  (See  section  A-A.)  The  southwest  limb  of  this 
anticline  has,  however,  been  sliced  off  by  the  displacement 
along  the  overthrust  of  the  Pinole  fault,  which  brings  the 
Tejon  against  several  horizons  of  the  folded  Monterey  strata. 
Tlie  northwest  end  of  the  Bear  Creek  anticline  is  truncated  by 
the  Pinole  fault,  and  its  southeast  end  is  broken  and  crushed 
by  the  Briones  overthrust  fault. 

Northeast  of  Pinole  Valley  and  north  of  the  end  of  the 
Bear  Creek  anticline  is  the  well-defined  Pinole  Ridge  anticline, 
which  is  also  truncated  and  offset  by  the  Pinole  fault.  The 
axis  of  this  fold  is  well  marked  for  about  3  miles  southeast  of 
the  Pinole  fault,  but  beyond  this  stretch  the  fold  dies  out. 
The  Pinole  Ridge  anticline  extends  west  of  the  Pinole  fault 
for  another  mile,  to  the  place  where  it  passes  beneath  the 
unconformably  overlying  Orinda  formation.  The  axis  of  the 
fold  is,  however,  distinctly  offset  by  the  fault.  Between  this 
anticline  and  the  northern  part  of  the  Bear  Creek  anticline 
lies  a  synclinal  fold,  which  is  also  transected  by  the  Pinole 
fault. 

Between  the  Bear  Creek  and  Pinole  Ridge  anticlines  on  the 
southwest  and  the  Franklin  fault  on  the  northeast  there  is  a 
broad  syncline  of  Monterey  formations,  in  the  center  of  which 


135 

lie  irregularly  shaped  remnants  of  the  San  Pablo  formation. 
(See  section  A-A.)  This  syncline  persists  halfway  across  the 
Concord  quadrangle,  although  it  is  traversed  by  the  Briones 
fault,  wliich  offsets  its  constituent  formations.  It  may  be 
appropriately  named  the  Briones  Hills  syncline.  The  fold 
terminates  against  the  Franklin  thrust  fault  near  the  town  of 
Walnut  Creek,  after  being  offset  again  by  a  later  transverse 
fault. 

Northeast  of  the  Franklin  fault,  in  the  northern  part  of  the 
Concord  quadrangle,  is  a  massive  anticline,  known  as  the 
Martinez  anticline,  which  exposes  the  Chico  formation  for  a 
breadth  across  the  strike  of  about  2  miles.  The  Chico  rocks 
are  flanked  in  normal  sequence  by  the  Martinez  and  Tejon 
strata,  but  on  the  southwest  flank  of  the  fold  these  are  succes- 
sively, from  south  to  north,  sliced  away  by  the  Franklin 
thrust,  so  that  finally  the  Chico  rocks  near  the  Santa  Fe  tunnel 
rest  directly  upon  the  Monterey.  The  complementary  syn- 
cline northeast  of  the  Martinez  anticline  is  known  as  the 
Pacheco  syncline,  a  simple  fold  in  the  middle  of  which  lie  the 
upper  beds  of  the  Monterey  group.  Its  axis  has  a  straight 
northwest  course  extending  from  Ygnacio  Valley  beyond  the 
northern  limits  of  the  quadrangle.  (See  sections  A-A  and 
B-B.) 

At  the  south  end  of  Ygnacio  Valley,  near  the  village  of 
Walnut  Creek,  is  another  deep  synclinal  fold,  known  as  the 
Walnut  Creek  syncline.  This  trough  lies  between  the  Frank- 
lin overthrust  fault  and  the  Arroyo  de  las  Nueces  y  Bolbones. 
It  is  an  overturned  syncline,  the  outcrops  on  its  limbs  diverg- 
ing southeastward  in  the  direction  of  the  pitch  of  the  fold. 
In  the  middle  of  the  trough  lies  the  Orinda  formation  and 
outward  from  this  on  either  flank  lie  the  other  formations  in 
normal  sequence  down  to  and  including  the  Tejon.  The  syn- 
cline is  thus  stratigraphically  symmetric  but  structurally  asym- 
metric, owing  to  overturning  to  the  southwest.  (See  section 
D-D.)  The  axial  plane  of  the  fold  dips  northeastward  and 
the  axis  pitches  southeastward.  On  the  southwest  limb  of  the 
fold  the  formations  are  in  their  normal  superposition,  with 
northeasterly  dip,  but  on  the  northeast  limb,  the  San  Pablo 


136 


rests  upon  the  Orinda,  the  Monterey  upon  the  San  Pablo,  and 
the  Tejon  upon  the  Monterey,  the  reversed  dip  being  com- 
monly about  45°  but  in  places  as  low  as  30°.  This  over- 
turned syncline  extends  far  beyond  the  Concord  quadrangle 
and  is  one  of  the  dominant  structural  features  of  the  southwest 
flank  of  Mount  Diablo.  Beneath  this  overturned  fold  lies  the 
thrust  plane  of  the  Franklin  fault.  It  is  evident  that  the 
great  pressure,  which  found  partial  relief  first  in  the  folding  of 
the  strata  and  next  in  the  overturning  of  the  fold,  found 
further  and  more  complete  relief  in  the  rupture  and  over- 
thrusting  manifested  in  the  Franklin  thrust  fault. 

Between  the  Franklin  fault  and  Las  Trampas  fault  is  an 
extended  syncline,  which  passes  through  Lafayette.  This  is 
truncated  at  its  northwest  end,  on  Bear  Creek,  by  the  Pinole 
fault  and  at  its  southeast  end  by  the  Franklin  fault.  It 
involves  the  Monterey,  San  Pablo,  and  Orinda  strata,  as  shown 
in  section  D-D.  This,  the  Lafayette  syncline,  is  flanked  on 
the  north  by  a  rather  sharp  anticline,  which  extends  from  the 
Franklin  fault  near  Walnut  Creek  through  Reesley  Valley 
to  the  Briones  fault  and  for  some  distance  beyond.  This,  the 
Reesley  Valley  anticline,  appears  to  have  a  low  pitch  to  the 
southeast.  On  the  south  side  of  the  Lafayette  syncline  is  a 
more  open  anticline,  which  extends  southeastward  from  the 
Pinole  fault  at  Bear  Creek  and  pitches  rather  steeply  in  that 
direction.  Between  the  Reesley  Valley  anticline  and  the  south 
end  of  the  Bear  Creek  anticline  is  a  short  but  well-defined 
syncline  pitching  southeastward  toward  and  abutting  upon  the 
Briones  fault  at  its  sharp  bend.  There  are  also  a  minor  syn- 
cline and  anticline  between  the  Bear  Creek  anticline  and  the 
San  Pablo  syncline  on  the  southwest  flank  of  Sobrante  Ridge. 

South  of  the  Bolinger  fault  is  a  large  anticline  which  has 
been  overthrust  northeastward  upon  a  syncline  of  the  Orinda 
formation  that  occupies  Bolinger  Canyon.  The  northwest  limb 
of  this  syncline  has  been  cut  off  by  the  fault,  and  the  northeast 
limb  of  the  anticline  has  been  buried  by  the  overriding  mass. 

In  general,  the  folding  and  the  faulting  in  the  Berkeley 
Hills  block  are  related  in  the  sense  that  they  are  both  mani- 
festations of  the  same  compressive  forces,  but  most  of  the  fault- 


137 


ing  was  later  than  the  folding  and  marked  the  culmination  of 
the  movement  caused  by  the  compressive  forces.  The  most 
noteworthy  general  fact  is  that  the  overthrusting,  whether  mani- 
fested in  overturned  folds  or  in  thrust  faults,  is  not  all  in  the 
same  direction.  The  great  thrust  movement  which  appears  in 
the  Walnut  Creek  syncline  and,  in  more  intense  form,  in  the 
Franklin  thrust  fault  is  clearly  due  to  a  force  that  acted  from 
the  northeast  toward  the  southwest.  Along  this  general  line 
of  deformation  Mount  Diablo  has  been  shoved  to  the  south- 
west, the  older  rocks  having  been  pushed  up  over  the  younger. 
It  is  equally  clear  that  the  overturning  of  the  Siesta  syncline 
between  Bald  Peak  and  Moraga  Valley  and  the  overthrusting 
on  the  Bolinger  and  Las  Trampas  thrust  faults  are  due  to 
similar  forces  acting  from  the  southwest. 

The  date  of  the  folding  and  associated  faulting  may  be 
determined  approximately  by  the  fact  that  not  only  the  Orinda 
formation  but  also  the  overlying  volcanic  rocks  and  fresh-water 
beds  of  the  Berkeley  group  have  participated  in  these  move- 
ments, even  in  their  most  acute  manifestations.  The  correla- 
tion of  the  Orinda  with  the  Merced  epoch  indicates  clearly 
that  the  principal  deformation  of  the  region  as  expressed  in 
folds  and  certain  associated  faults  of  the  Tertiary  formations 
was  post-Pliocene  in  age  and  may  therefore  be  referred  with 
confidence  to  the  general  epoch  of  mountain  making  which 
ended  the  Tertiary  and  began  the  Quaternary  throughout  the 
California  region.  The  faulting  that  tilted  and  separated  the 
great  orogenic  blocks,  however,  occurred  later,  in  Quaternary 
time. 

It  should  be  noted  that  there  is  evidence  of  an  earlier 
and  milder  deformation  of  the  Berkeley  Hills  region  in  the 
unconformity  between  the  Orinda  and  the  San  Pablo.  In 
the  normal  sequence  the  Orinda  rests  upon  the  San  Pablo, 
but  over  considerable  areas  the  San  Pablo  had  been  removed 
by  erosion  before  the  deposition  of  the  Orinda,  which  therefore 
rests  upon  the  upturned  edges  of  the  several  divisions  of  the 
Monterey.  Inasmuch  as  the  structural  discordance  between 
the  San  Pablo  and  the  Monterey  is  not  pronounced,  it  seems 
probable    that   the    pre-Orinda    folding   was    post-San    Pablo. 


138 


There  is  also  clear  evidence  of  uplift  and  erosion  of  the  Eocene 
formations  prior  to  the  deposition  of  the  Monterey,  but  the 
angular  discordance  is  not  great.  Similarly,  there  was  but 
slight  deformation  in  the  interval  between  Cretaceous  and 
Eocene  time.  In  the  interval  between  Franciscan  and  Creta- 
ceous time,  however,  there  was  notable  and  widespread  dis- 
turbance, uplift,  and  erosion.  It  therefore  appears  that  of  the 
numerous  and  various  earth  movements  that  have  affected  the 
region  since  Franciscan  time  the  movement  at  the  close  of 
the  Tertiary  period  was  the  most  intensely  deformational  in  its 
effects. 

GEOLOGIC  HISTORY. 

PRE-FRANCISCAN    TIME. 

The  geologic  history  of  the  area  described  in  this  folio  begins 
with  the  story  told  by  the  Gavilan  limestone  of  Pilarcitos 
Canyon.  This  limestone  is  included  in  the  quartz  diorite  and 
is  but  a  fragment  of  a  series  of  marbles,  quartzites,  and  crystal- 
line schists  that  occur  more  abundantly  in  other  parts  of  the 
Coast  Ranges,  particularly  farther  south.  These  rocks  are  the 
highly  metamorphosed  representatives  of  sediments  of  unknown 
age,  which  were  deposited  in  this  region  long  before  the  advent 
of  the  quartz  diorite  batholith  that  now  forms  a  large  part  of 
the  southern  Coast  Ranges. 

The  intrusion  of  the  quartz  diorite  into  these  ancient  sedi- 
mentary rocks  was  doubtless  the  culminating  event  of  a  series 
of  profound  disturbances  that  formed  a  high  mountain  chain. 
Both  the  quartz  diorite  and  the  rocks  of  the  sedimentary  series 
into  which  it  is  intruded  extend  without  material  change  of 
character  or  apparent  structural  break  from  the  Coast  Ranges 
to  the  Sierra  Nevada.  If  followed  northward  into  the  Sierra 
Nevada  they  appear  to  be  continuous  witli  the  granite  and  the 
metamorphic  rocks,  respectively,  of  the  parts  of  that  range 
wliich  are  described  in  several  folios  of  the  Geologic  Atlas  of 
the  United   States."     In  these  folios  the  granitic  rocks  have 

"U.  S.  Geol.  Survey  Geol.  Atlas,  folios  8,  5,  11,  15,  17,  18,  29,  31,  37,  89,  41, 
43,  51,  63,  66,  and  138. 


139 

been  shown  to  be  post-Jurassic  in  age  and  the  invaded  rocks 
to  be  in  part  Upper  Jurassic.  It  therefore  appears  that  the 
bedrock  complex  of  the  Sierra  Nevada  extends  into  the  Coast 
Ranges,  and  that  if  the  granitic  rocks  of  the  Sierra  Nevada 
are  post-Jurassic  the  granitic  rocks  of  the  Coast  Ranges  are  also 
presumably  post-Jurassic. 

FRANCISCAN    EPOCH. 

The  high  mountain  chain  that  was  formed  in  the  region  of 
the  Coast  Ranges  by  the  disturbances  that  culminated  in  the 
intrusion  of  the  granitic  rocks  was  subjected  to  long-continued 
erosion  and  was  greatly  degraded  before  it  was  submerged  to 
receive  the  Franciscan  sediments.  The  character  of  the  forma- 
tions of  the  Franciscan  group  indicates  a  migration  to  and  fro 
of  the  shore  line  of  the  Franciscan  sea.  The  foraminiferal 
limestones  and  the  radiolarian  cherts  were  probably  deposited 
on  a  sea  floor  at  places  so  far  from  the  shore  as  to  preclude 
large  admixtures  of  terrigenous  sediments,  whereas  the  sand- 
stones of  the  same  series  are,  if  marine  at  all,  clearly  littoral 
deposits.  Tiie  migration  of  the  shore  line  thus  indicated  by 
the  alternation  of  sandstone  with  either  limestone  or  radiola- 
rian chert  is  only  an  expression  of  a  vertical  oscillation  of  the 
land  in  respect  to  sea  level  if  the  sea  level  was  practically 
stationary,  and  its  deviation  from  a  stationary  position  was 
probably  not  great  enough  to  weaken  the  conclusion  that  in 
Franciscan  time  the  region  was  subject  to  alternating  uplift 
and  depression.  As  the  Franciscan  group  includes  at  least 
three  nonterrigenous  formations  that  are  separated  by  detrital 
formations  the  stratigraphy  indicates  at  least  three  notable 
depressions  of  the  region  in  Franciscan  time  and  three  move- 
ments of  uplift. 

Certain  sheets  of  amygdaloidal  lava  that  are  interstratified 
with  the  sandstones  of  the  Franciscan  group  show  that  during 
part  of  Franciscan  time  volcanoes  were  active  and  that  their 
lavas  were  poured  out  on  the  area  of  deposition  on  which  the 
sandstones  were  accumulating. 

The  Franciscan  epoch  of  sedimentation  was  brought  to  a 
close  by  the  uplift  and  deformation  of  the  region.     A  feature 


140 

of  tlie  disturbance  was  the  intrusion  into  the  Franciscan  strata 
at  many  places  of  irregular  bodies  of  basalt  and  diabase  which 
show  ellipsoidal  or  spheroidal  structure.  This  intrusion  was 
followed  by  the  intrusion  of  peridotites  in  the  form  of  lacco- 
lithic  lenses  and  dikes.  These  peridotites  have  since  been 
altered  to  the  serpentine  which  is  so  common  in  the  Franciscan 
rocks  of  the  Coast  Ranges.  Closely  associated  with  these  intru- 
sive bodies  of  ellipsoidal  basalt  and  serpentinized  peridotite  are 
belts  and  irregular  areas  of  glaucophane  schists  and  other 
metamorphic  rocks.  These  schists  are  metamorphosed  igneous 
and  sedimentary  rocks,  and  their  mode  of  occurrence  indicates 
that  they  are  a  product  not  of  dynamic  metamorphism  but  of 
contact  action.  This  metamorphism  took  place  in  the  Fran- 
ciscan rocks  before  the  deposition  of  the  Knoxville  formation 
and,  indeed,  before  the  erosion  ijiterval  between  Franciscan 
and  Knoxville  time.  From  this  it  appears  that  the  intrusions 
of  the  peridotite  and  ellipsoidal  basalt  must  have  occurred  so 
soon  after  the  deposition  of  the  Franciscan  strata  that  they 
may  with  propriety,  as  well  as  convenience,  be  referred  to 
Franciscan  time. 

KNOXVILLE   EPOCH. 

The  erosion  which  followed  the  uplift  of  the  Franciscan 
rocks  appears  nowhere  to  have  entirely  removed  them.  The 
degradation  and  removal  of  the  uplifted  Franciscan  was  inter- 
rupted by  the  depression  that  ushered  in  the  Cretaceous  period 
of  sedimentation.  The  basal  formation  of  the  Cretaceous  is 
the  Knoxville,  which  in  this  area  is  composed  largely  of  shale. 
The  occurrence  of  only  small  deposits  of  conglomerate  or  other 
coarse  detrital  beds  at  the  base  of  the  Knoxville  and  the  entire 
absence  of  such  beds  in  many  sections  show  that  peculiar  con- 
ditions attended  the  beginning  of  the  Cretaceous  depression. 
The  coarse  sandstones  and  conglomerates  which  are  usually 
spread  over  a  sinking  continental  slope  by  a  transgressing  sea 
are  here  but  very  feebly  if  at  all  represented.  The  facts 
observed  indicate  that  the  profile  of  the  coast  of  the  Knoxville 
sea  was  very  flat  and  that  the  streams  which  flowed  into  that 
sea  were  of  low  grade,  carrying  chiefly  fine  sediment.     A  slight 


141 


subsidence  of  a  coast  so  low  would  produce  broad  expanses  of 
shallow  lagoons  and  swamps  devoid  of  powerful  currents.  In 
such  shallow,  rehitively  stagnant  water  the  Knoxville  appears 
to  have  been  deposited. 

But  if  the  land  surface  that  subsided  and  received  the 
Knoxville  deposits  was  low  and  flat,  the  low  relief  must 
have  been  due  to  peneplanation  in  post-Franciscan  time,  so 
that  the  interval  between  Franciscan  and  Knoxville  time  was 
long.  Further,  the  post-Franciscan  disturbance  could  not 
have  involved  any  large  vertical  displacements,  for  if  such  dis- 
placements had  occurred  the  peneplanation  of  the  region  would 
have  completely  removed  the  Franciscan  from  the  uplifted 
blocks,  so  that  the  Knoxville  would  have  been  deposited 
upon  pre-Franciscan  rocks,  but  the  Knoxville  generally  rests 
upon  the  Franciscan.  The  fact  that  the  Knoxville  beds  attain 
a  very  considerable  thickness  in  neighboring  portions  of  the 
Coast  Ranges  indicates  that  the  sea  floor  continued  to  subside 
steadily  during  Knoxville  time  while  the  adjoining  land,  from 
which  the  sediments  were  drawn,  suffered  no  marked  uplift  or 
steepening  of  its  gentle  slopes. 

CHICO    EPOCH. 

In  most  of  the  area  mapped  the  Knoxville  formation  is 
succeeded  by  the  Oakland  conglomerate  member  of  the  Chico 
formation.  This  member  reaches  a  maximum  thickness  of 
about  1000  feet  and  in  places  is  rather  coarse,  containing 
waterworn  bowlders  6  to  10  inches  in  diameter,  although  the 
dimensions  of  most  of  the  pebbles  are,  of  course,  nuich  less  than 
these,  the  smaller  ones  being  of  the  sizes  of  marbles  and  peas. 
This  conglomerate  is  interesting  and  significant  and  would 
ordinarily  be  regarded  as  indicating  an  unconformity,  but  this 
member  appears  to  rest  concordantly  upon  the  Knoxville  for- 
mation, the  relation  showing  that  the  sea  floor  upon  which  the 
Knoxville  beds  were  accumulating  was  probably  not  uplifted 
at  the  beginning  of  the  deposition  of  the  conglomerate.  The 
thickness  of  the  conglomerate  indicates  that  during  its  accumu- 
lation there  was  progressive  subsidence  to  an  amount  measured 
by  that  tlr3knes3.     But  if  the  subsidence  of  the  Knoxville  sea 


142 

floor  continued  into  Oakland  time  without  reversal  the  sudden 
change  in  the  character  of  the  sediments  shows  that  the  region 
from  which  these  sediments  were  drawn  was  changed  at  the 
close  of  Knoxville  time  from  a  low-l^^ing  country,  with  low- 
grade  streams,  to  an  upland  dissected  by  actively  eroding- 
torrents.  The  Knoxville  subsidence  was  widespread  and  may 
be  regarded  as  an  epeirogenic  movement,  and  the  Oakland 
conglomerate  may  be  regarded  as  the  depositional  record  of 
an  orogenic  movement  which  deformed  the  coast  of  the  Creta- 
ceous sea  but  which  only  slightly  affected  the  adjacent  sea 
bottom. 

The  Oakland  conglomerate  member  is  succeeded  by  a  great 
thickness  of  sandstones  and  shales,  with  scattered  lenses  of 
conglomerate,  which  comprise  the  rest  of  the  Chico  formation. 
The  prevalence  of  fine-grained  sandstones,  the  notable  dearth 
of  limestones,  and  the  scarcity  of  conglomerates  clearl}^  indicate 
tliat  the  abrupt  profiles  which  characterized  the  continental 
margin  in  Oakland  time  had  been  greatly  subdued  and  that 
the  later  Chico  sea  remained  shallow  though  steadil}-  subsiding. 

MARTINEZ    EPOCH. 

The  advent  of  the  Tertiary  appears  to  have  been  marked  by 
no  violent  orogenic  movements,  but  there  is  evidence  of  general 
warping  and  uplift  of  the  floor  of  the  Chico  sea  and  of  an 
interval  of  erosion.  In  the  quadrangles  described  in  this  folio 
no  angular  discordance  between  the  early  Eocene  strata  and 
the  underlying  Chico  has  been  detected,  but  in  the  adjoining 
Mount  Diablo  quadrangle  there  is  a  well-defined  unconformity 
between  the  Martinez  and  the  Chico,"  and  this  unconformity 
doubtless  prevails  over  the  Concord  quadrangle,  although 
conditions  there  are  adverse  to  its  discovery.  The  Martinez 
rocks  of  San  Pedro  Point,  in  the  San  Mateo  quadrangle,  rest 
directl}^  upon  a  pre-Knoxville  surface,  yet  on  the  coast  a  little 
farther  south,  in  the  Santa  Cruz  quadrangle,  thei'e  are  several 
thousand  feet  of  Chico  strata.  It  seems  probable  that  the  Chico 
beds  once  extended  over  the  San  Francisco  Peninsula  and  were 


"Dickerson,  R.  E.,  California  Univ.  Dept.  Geology  Bull.,  vol.  6,  No.  8, 

1911. 


I4;j 


thus  continuous  on  both  sides  of  the  Bay  of  San  Francisco.  If 
they  were  so  deposited  there  must  have  been  a  long  period  of 
degradation  in  post-Chico  time  to  permit  the  deposition  of 
the  Martinez  beds  of  San  Pedro  Point  upon  a  pre-Chico 
surface.  This  presumption  of  a  considerable  interval  of  erosion 
between  the  Cretaceous  and  the  Tertiary  is  consistent  with  the 
abrupt  change  of  the  fossil  fauna  from  one  terrane  to  the  other. 
The  fact  that  the  Martinez  formation  is  not  so  widely  distrib- 
uted as  the  Cliico  indicates  that  the  marine  basin  of  sedimenta- 
tion was  smaller  in  early  Tertiary  time  than  it  was  before  the 
uplift  at  the  close  of  the  Chico. 

TEJON    EPOCH. 

After  more  than  2000  feet  of  sandstones  and  shales  had 
accumulated  in  Martinez  time  the  region  again  suffered  dis- 
turbance and  uplift.  The  denudation  begun  by  this  movement 
appears  to  have  been  moderate,  for  although,  as  Dickerson"  has 
shown,  an  unconformity  exists  between  the  Martinez  and  the 
overlying  Tejon  in  the  Mount  Diablo  quadrangle,  no  discord- 
ance at  this  contact  has  been  detected  in  the  Concord  quadran- 
gle except  in  so  far  as  it  may  be  inferred  from  the  conglomerate 
at  the  base  of  the  Tejon  northwest  of  Walnut  Creek.  There 
is,  however,  a  pronounced  contrast  in  the  fossil  faunas  of  the 
two  formations.  The  Tejon  subsidence  reestablished  in  a  gen- 
eral way  the  conditions  that  had  prevailed  in  Martinez  time, 
but  the  Tejon  area  of  sedimentation  was  probably  larger  than 
that  of  the  Martinez.  The  sea  was  shallow  and  the  subsidence 
permitted  the  deposition  of  about  2000  feet  of  sandstones  and 
shales. 

MONTEREY    ErOCII. 

The  Tejon  subsidence  and  sedimentation  was  brought  to  a 
close  by  earth  movement  and  uplift,  and,  after  erosion,  a  wide- 
spread submergence  ushered  in  the  period  of  deposition  of  the 
Monterey  group.  The  interval  between  the  Tejon  and  the 
Monterey  was  probably  a  notable  period  in  the  geologic  history 

"Op.  eit. 


144 


of  the  region.  The  fects  presented  under  the  heading  "Mon- 
terey group"  indicate  that  large  areas  of  pre-Monterey  rocks 
were  raised  above  the  sea  and  somewhat  fokled  and  faulted  by 
the  uplift  and  that  these  areas  were  extensively  eroded  before 
the  Montere}^  submergence  began,  so  that  the  Monterey  rocks 
lie  unconformably  upon  the  Tejon  and  older  formations,  the 
discordance  in  dip  being  in  places  considerable. 

The  fact  of  chief  historic  interest  connected  with  the  Mon- 
terey is  the  record  of  crustal  oscillation  revealed  by  its  con- 
stituent formations.  Even  in  the  Concord  quadrangle  the 
sequence  of  the  Monterey  formations  is  not  uniform.  The 
most  complete  section,  containing  not  only  the  greatest  thick- 
ness of  strata  but  also  the  largest  number  of  petrographically 
distinct  formations,  is  found  in  the  large  sj^ncline  that  lies 
between  the  Bear  Creek  anticline  and  the  Franklin  overthrust 
fault.  Here  five  divisions  of  sandstone  alternate  with  four 
divisions  of  bituminous  shale,  as  described  in  detail  under  the 
heading  "Monterey  group."  In  their  purer  facies  the  shales 
are  nonterrigenous  deposits,  though  in  places  they  include  silt 
or  even  fine  sand,  and  they  were  evidently  deposited  rather  far 
from  the  shore,  but  the  sandstones  between  the  shales  and 
those  at  the  bottom  and  top  of  the  Monterey  group  are  littoral 
deposits. 

The  migration  of  the  shore  line  to  and  fro  implied  by  this 
alternation  in  conditions  of  sedimentation  is  interpreted  to 
signify  a  vertical  oscillation  of  the  region.  Apparently  four 
movements  of  depression  and  four  of  uplift  occurred  during 
Monterey  time,  but  the  net  result  of  these  upward  and  down- 
ward movements  was  a  depression,  which  corresponded  approx- 
imately in  amount  to  tlie  entire  thickness  of  the  Monterey 
group  in  this  region.  Although  this  oscillation  so  greatly 
affected  the  depth  of  tlie  water  and  tlie  position  of  the  shore 
tliat  it  raiHcally  clianged  the  conditions  of  deposition  in  the 
region  indicated,  it  may  not  have  affected  these  conditions  in 
the  neighboring  parts  of  the  sea  floor  that  lay  farther  from 
tlie  continental  margin,  so  that  the  deposition  of  bituminous 
shales  there  may  have  proceeded  continuously.  Thus  a  uni- 
form body  of  bituminous  shales  would  become  the  stratigraphic 


145 


and  chronologic  equivalent  of  a  series  of  alternating  sandstones 
and  shales  nearer  shore.  Conversely,  the  margin  of  the  basin 
might  have  remained  a  littoral  region  notwithstanding  depres- 
sions that  affected  chiefly  the  rest  of  the  -basin,  so  that  sand- 
stones may  have  been  continuously  deposited  along  the  shore 
as  the  stratigraphic  equivalent  of  the  alternating  sandstones 
and  shales  laid  down  farther  out. 

Conditions  favoring  the  continuous  deposition  of  bituminous 
shale  after  the  transgressional  basal  sandstone  and  conglomerate 
had  been  formed  were  probably  realized  in  the  Monterey  of 
the  Point  Reyes  Peninsula  and  probably  also  in  the  Monterey 
of  Monterey  County.  The  second  condition — that  of  continu- 
ous littoral  sedimentation — is  in  some  measure  realized  in  the 
Monterey  of  the  Pacheco  syncline  and  Shell  Ridge.  At  both 
of  these  places  we  find  the  lower  and  upper  portions  of  the 
Monterey,  but  the  deep-water  sediments  of  the  middle  Monterey 
are  represented  by  a  single  comparatively  thin  and  impure 
formation  of  bituminous  shale. 

SAN  PABLO  EPOCH. 

Monterey  time  was  brought  to  a  close  by  uplift  and  disturb- 
ance of  the  Coast  Range  region.  The  disturbance  was  great 
in  Santa  Cruz  County,  where,  as  W.  F.  Jones"  has  sbown,  the 
San  Pablo  formation  rests  upon  the  worn  edges  of  the  nearly 
vertical  Monterey  strata  and,  moreover,  has  a  basal  conglom- 
erate. The  discordance,  though  not  so  pronounced,  is  still  well 
marked  in  the  Mount  Diablo  quadrangle.''  There  is  also  dis- 
tinct indication  of  discordance  between  the  San  Pablo  and 
Monterey  in  the  distribution  of  the  San  Pablo  along  the  middle 
of  the  large  syncline  in  the  northwestern  part  of  tlie  Concord 
quadrangle.  Elsewhere  in  the  Concord  quadrangle,  however, 
the  San  Pablo  follows  the  Monterey  without  appreciable  dis- 
cordance. ■  The  subsidence  that  permitted  the  deposition  of  the 
San  Pablo  beds  appears  to  have  formed  a  much  smaller  basin 
than  that  which  existed  in  Monterey  time.     The  prevailingly 

"California  Univ.  Dept.  Geology  Bull.,  vol.  0,  No.  3,  1911. 
^Clark,  B.  L.,  The  Neocene  section  at  Kirker  Pass,  on  the  north  side  of 
Mount  Diablo:   Idem,  vol.  7,  No.  4,  pp.  47-60,  1912. 

San  Fraucisc'o — lO 


146 

thick  bedding  and  rather  coarse  grain  of  the  sandstones  of 
tlie  San  Pablo  and  their  general  blue  color  tend  to  mark  oflf 
this  formation  from  the  underlying  commonly  light-colored  or 
rusty-yellow  sandstones  of  the  Monterey  and  indicate  a  change 
in  the  conditions  of  deposition.  The  absence  of  the  bitumi- 
nous shales  that  so  strongl}^  characterize  the  Monterey  also 
points  to  such  a  change. 

MERCED    EPOCH. 

The  superposition  of  the  Merced  formation  upon  the  San 
Pablo  has  nowhere  been  observed  in  the  area  here  mapped,  but 
such  superposition,  with  evidence  of  unconformity,  occurs  near 
Chittenden,  in  Santa  Cruz  County,  and  has  been  described  by 
W.  F.  Jones."  The  absence  of  the  San  Pablo  below  the  Merced 
in  the  San  Mateo  and  Tamalpais  quadrangles  itself  suggests  an 
unconformity  between  the  two,  unless  the  San  Pablo  basin  was 
so  small  that  it  did  not  cover  this  part  of  the  field. 

The  depression  wdiich  permitted  the  accumulation  of  Merced 
strata  to  the  thickness  of  over  a  mile  on  the  site  of  the  present 
San  Francisco  Peninsula  appears  to  have  been  local  and  to  have 
formed  a  sinking  trough  in  which  there  accumulated  marine 
clays,  sands,  and  conglomerates.  The  land  surface  that  was 
thus  gradually -depressed  moi'e  than  a  mile  below  sea  level  had 
supported  a  forest  growth,  represented  by  abundant  pine  cones, 
which  occur  in  the  basal  bed  of  the  series  at  Mussel  Rock.  The 
fresh-water  Orinda  formation,  which  is  partly  fluviatile  and 
partly  lacustrine,  lies  in  a  similar  geosynclinal  trough  east  of 
the  Berkeley  Hills — a  trough  that  must  have  subsided  as  the 
formation  was  deposited.  These  two  deposits  were  probably 
synchronous  in  origin,  for  the  basal  beds  of  the  marine  Merced 
fornjation  in  Sonoma  County,  described  by  Osmont,''  contain 
abundant  white  tuff,  which  extends  southward  to  the  Bay  of 
San  Francisco  and  is  probably  identical  with  the  Pinole  tuff. 
This  tuff  occurs  also  in  the  basal  beds  of  the  Orinda  formation 
in  the  northeastern  part  of  the  San  Francisco  quadrangle 
and  is  thus  common  to  the  marine  Merced  and  the  fresh-water 

"California  Univ.  Dept.  Geology  Bull.,  vol.  6,  No.  3,  1911. 
''Ideni,  vol.  4,  No.  3,  1904. 


147 

Orinda.  The  barrier  between  the  marine  trough  and  the  fresh- 
water trough  thus  formed  appears  to  have  been  nearly  coinci- 
dent with  the  present  Kne  of  the  Berkeley  Hills  but  may 
have  lain  a  little  west  of  it. 

Volcanic  ash  occurs  in  beds  high  up  in  both  the  Merced  and 
Orinda  and  was  probably  ejected  from  distant  volcanoes  that 
were  intermittently  active.  The  ash  in  the  Merced  appears  to 
be  a  fresh  white  andesitic  pumiceous  glass,  but  that  in  the 
Orinda  is  thoroughly  decomposed. 

BERKELEY    EPOCH. 

At  the  close  of  Orinda  time  a  slight  depression  probal)ly 
occurred  on  the  southwest  side  of  the  structural  trough  in 
which  the  fresh- water  sediments  of  that  epoch  accumulated,  and 
into  this  depression  there  was  poured  a  succession  of  andesitic 
and  basaltic  lavas  and  occasional  showers  of  ashes.  Between 
these  volcanic  flows  and  showers  there  were  times  W'hen  fluvia- 
tile  and  lacustral  conditions  recurred  and  other  times  in  which 
the  lava  was  exposed  to  atmospheric  oxidation  or  to  erosion. 
The  beds  of  tuff  or  ash  that  are  intercalated  with  the  lavas 
include  rhyolitic  tuff  but  no  flows  of  massive  rhyolite.  This 
accumulation  of  volcanic  and  interstratified  fresh-water  beds 
forms  the  Berkeley  group, 

DEFORMATION    AT    CLOSE   OF    TERTIARY    PERIOD. 

At  the  close  of  Berkeley  time,  Avliich  marked  also  the  end  of 
the  Tertiary  period,  the  Merced,  Orinda,  and  Berkeley  strata 
became  involved  with  all  the  older  rocks  in  the  great  earth 
movements  w4iich  deformed  the  region.  The  basement  upon 
which  the  Merced  had  been  deposited  and  which,  as  we  have 
seen,  had  been  depressed  more  than  a  mile  below  sea  level, 
was  lifted  far  above  sea  level,  as  may  be  plainly  seen  at  the 
exposure  near  INIussel  Kock,  in  the  San  Mateo  quadrangle. 
The  extent  of  this  uplift  shows  the  magnitude  of  the  orogenic 
movements  that  closed  the  Tertiary  period.  The  overturning 
of  the  syncline  involving  the  Berkeley  group  near  Moraga 
Valley  and  the  overturning  of  the  Orinda  beds  in  the  oppo- 
site  direction    in    the   Walnut    Creek    svncline    indicate   the 


148 


intensity  of  the  action.  Most  of  tlie  folding  and  faulting 
shown  on  the  structure-section  sheets  of  this  folio  occurred  at 
this  time. 

CAMPUS    EPOCH. 

After  a  partial  degradation  of  the  Siesta  syncline  another 
fresh-water  basin  was  formed  across  the  eroded  edges  of  the 
upturned  lavas  of  the  Berkeley  group,  and  in  this  apparently 
rather  small  basin  accumulated  the  gravels,  clays,  tuffs,  and 
lavas  of  the  Campus  formation. 

ALAMEDA    EPOCH. 

After  the  Campus  basin  had  been  filled  but  before  the 
Berkeley  Hills  had  been  uplifted  the  degradation  of  the  uplands 
continued,  and  in  the  depressions  there  accumulated  the  Ala- 
meda formation,  which  is  chiefly  a  sandy  clay  with  interca- 
lated delta  gravels  derived  from  Alameda  Creek  and  a  few 
beds  of  sand  containing  marine  shells,  all  indicating  that  parts 
of  the  region  which  are  now  sharply  accentuated  were  then 
of  moderate  relief. 

POST-ALAMEDA    DIASTEOPHISM. 

After  the  deposition  of  the  Alameda  formation  a  movement 
occurred  that  separated  the  San  Francisco-Marin  block  from 
the  Berkeley  Hills  block  and  probably  also  from  the  Montara 
block.  This  movement  consisted  chiefly  of  faulting  and  mono- 
clinal  tilting,  one  result  of  which  was  the  outlining  of  the  valley 
system  of  the  Bay  of  San  Francisco.  A  considerable  interval 
had  tlius  elapsed  between  the  diastrophism  which  closed  the 
Tertiary,  exemplified  in  the  folding  and  faulting  of  the  Merced 
and  Orinda,  and  this  later  movement,  which  brought  the  tilted 
blocks  into  their  present  positions  and  left  the  Campus  rocks 
in  the  form  of  steps  on  the  steep  west  front  of  the  Berkeley 
Hills  block.  This  final  uplift  left  the  fine  Alameda  sediments 
close  to  the  base  of  the  same  steep  front,  a  situation  in  which 
they  evidently  could  not  have  been  originally  deposited. 


149 


SAN    ANTONIO    EPOCH. 


In  the  fault  valley  of  the  Bay  of  San  Francisco  were 
deposited  later  Quaternary  sediments.  The  earliest  of  these  is 
the  San  iVntonio  formation,  a  series  of  fans  of  coarse  alluvium 
spread  out  at  the  base  of  the  Berkeley  Hills  and  made  up  of 
detritus  derived  from  the  newly  formed  steep  front  of  the 
Berkeley  Hills  block.  That  the  San  Antonio  formation  is  the 
depositional  record  of  the  uplift  of  the  Berkeley  Hills  block  is 
clearly  shown  by  the  fact  that  for  a  considerable  portion  of  its 
extent  it  is  separable  into  two  parts.  The  lower  part  is  made 
up  of  detritus  brought  down  in  the  early  stages  of  the  degra- 
dation of  the  new  mountain  front,  before  the  canyons  had  cut 
back  to  the  chert  of  the  Monterey  group,  and  therefore  includes 
none  of  this  chert.  The  upper  part  abounds  in  chert  frag- 
ments derived  from  the  Monterey  and  represents  the  degrada- 
tion after  the  canyons  had  cut  back  to  the  belt  of  chert  which 
marks  for  some  distance  the  present  crest  of  the  uplifted  block. 
As  Alameda  Creek  in  its  transverse  passage  through  the 
Berkeley  Hills  in  the  gorge  near  Niles,  just  east  of  the  Hay- 
wards  quadrangle,  is  an  antecedent  stream,  formed  before  the 
uplift  of  the  Berkeley  Hills,  the  lower  beds  of  its  delta  or 
alluvial  fan  probably  correspond  to  the  iVlameda  formation 
and  the  upper  beds  to  the  San  Antonio  formation,  deposition 
having  been  continuous  from  one  epoch  into  the  other. 

The  remains  of  a  notable  vertebrate  fauna,  now  extinct,  in 
the  San  Antonio  formation,  particularly  in  its  occurrence  north 
of  the  San  Francisco  quadrangle,  represent  the  life  of  this 
epoch. 

MERRITT    AND    TEMESCAL    EPOCH. 

The  abrupt  cliff-like  edge  of  the  alluvial  embankment  of  the 
San  Antonio  formation  in  the  city  of  Oakland  indicates  that 
the  region  was  depressed  and  attacked  by  the  waves,  so  that 
a  wave-cut  terrace  and  sea  cliff  were  here  carved  out  of  it. 
During  this  period  of  depression  the  Merritt  sand  of  Oakland 
and  Alameda  was  deposited.  Since  then  the  dissection  of  the 
main  embankment  of  the  San  Antonio  formation  has  given  it  a 


150 


prononiiced  terraced  effect  and  lias  supplied  the  materials  of 
the  Teinescal  formation,  which  rests  upon  the  Merritt  sand  in 
the  city  of  Oakland.  Since  the  deposition  of  the  Temescal 
formation  the  region  has  been  slightly  uplifted  and  shallow 
valleys  have  been  cut  in  it  and  then  submerged  by  a  later 
subsidence,  which  produced  tlie  drowned  effects  in  the  topog- 
raphy about  Lake  Merritt  and  in  the  channel  between  Oakland 
and  Alameda,  but  these  later  movements  may  have  been  local. 
It  is  noteworthy  that  the  last  depression  appears  to  have  con- 
tinued down  to  the  time  of  the  occupation  of  the  region  by 
man.  The  base  of  a  large  shell  mound  near  Emery,  on  the 
Oakland  shore  of  the  bay,  is  more  than  2  feet  below  sea  level. 
The  bases  of  other  Indian  mounds  on  the  shores  of  the  bay 
are  also  ])elow^  sea  level,  and  as  they  were  all  probably  started 
on  dry  land,  their  present  position  indicates  very  recent 
subsidence." 

EECENT    UPLIFT    AND    DEPEJ:SSION. 

The  record  of  uplift  and  depression  which  is  read  in  the 
Quaternary  deposits  of  the  east  side  of  the  Bay  of  San  Fran- 
cisco and  in  their  dissection  is  not  matched  on  the  west  side. 
The  alluvium  which  extends  from  the  base  of  the  steep  slopes 
to  the  edge  of  the  salt  marshes  of  the  bay  is  the  equivalent  of 
the  Temescal  formation,  and  the  Alameda,  San  Antonio,  and 
Merritt  formations  are  absent.  The  dentate  contour  of  the 
shore — a  series  of  embay  men  ts  separated  by  rocky  promon- 
tories— which  characterizes  the  bay  side  of  the  Marin  Peninsula 
and  of  the  San  Francisco  Peninsula  as  far  south  as  San  Bruno 
Point  clearly  points  to  a  subsidence  much  more  pronounced 
than  that  which  occurred  on  the  east  side  of  the  bay.  If  the 
missing  formations  were  ever  deposited  on  the  west  shore,  and 
it  seems  probable  that  some  of  them  were,  they  have  been 
entirely  submerged  by  the  greater  subsidence  of  this  part  of 
the  bay. 

"Ulile,  JIux,  The  Kmeryville  shell  mound:  California  Univ.  Pub.  Am. 
Archeology  and  Ethnology,  vol.  7,  No.  1,  1907.  Nelson,  N.  C,  The  shell 
jiiounds  of  the  San  Franoiseo  Bay  region:  Idem,  No.  4,  1909. 


151 

The  heavy  bank  of  old  alluvium  on  the  west  side  of  Crystal 
Springs  Lake,  at  the  southern  border  of  the  San  Mateo  quad- 
rangle, which  is  the  northern  extension  of  the  Santa  Clara  for- 
mation of  the  Santa  Cruz  quadrangle,  is  the  product  of  degra- 
dation and  fluviatile  deposition  during  a  period  whose  geologic 
age  is  in  doubt. 

Outside  of  the  Golden  Gate  the  striking  wave-cut  terrace 
at  Bolinas  affords  unequivocal  evidence  of  uplift,  the  minimum 
measure  of  which  is  the  elevation  of  the  terrace  where  it  abuts 
upon  its  now  degraded  sea  cliff.  This  elevation  is  about  250 
feet.  The  uplift  thus  clearly  indicated  is  extremely  interesting 
in  view  of  the  absence  of  evidence  of  uplift  on  the  shore  of  the 
Marin  Peninsula  northeast  of  the  San  Andreas  rift  and  in  view 
of  the  positive  evidence  of  depression  and  the  entire  absence  of 
evidence  of  uplift  on  the  bay  side  of  the  Marin  Peninsula. 
Similarly  uplifted  shore  lines  are  absent  on  the  Pacific  side 
of  the  San  Francisco  Peninsula  between  the  Golden  Gate 
and  Point  San  Pedro.  South  of  this  point,  however,  partic- 
ularly south  of  Halfmoon  Bay  as  for  as  Santa  Cruz,  elevated 
strands  are  prominent  features  of  the  coastal  profile.  It  is 
noteworthy  also  that  on  the  coast  north  of  Tomales  Bay 
elevated  strands  again  become  conspicuous,  and  on  the  east 
side  of  Tomales  Bay  itself  there  are  raised  shell  beds,  which 
stand  20  to  30  feet  above  the  present  shore  line.  Since  the 
uplift  of  the  Bolinas  terrace  there  has  been  a  slight  depression 
of  the  Point  Reyes  Peninsula,  as  may  be  readily  inferred  from 
the  drowning  of  the  small  streams  flowing  into  Drake's  Bay, 
near  Point  Reyes  light.  The  evidence  thus  presented  shows 
that  the  diastrophic  movements  of  the  region  about  the  Bay  of 
San  Francisco  in  Quaternary  time  have  been  complex  and 
uneven.  The  Marin  Peninsula  proper  between  Bolinas  and 
the  Bay  of  San  Francisco  has  undergone  marked  depression, 
and  with  this  depression  may  be  associated  the  invasion  of 
the  Golden  Gate  by  the  sea.  The  east  side  of  the  bay  in 
the  vicinity  of  Oakland  and  Berkeley  has  been  alternately 
lowered  and  raised,  the  net  result  of  the  movements  being 
an  uplift.  The  Point  Reyes  Peninsula  has  certainly  l)een 
uplifted  more  than  250  feet  and  this  uplift  has  been  followed 


152 


by  a  slight  depression.  On  both  coasts  of  the  San  Francisco 
Peninsula  the  evidence,  though  not  so  clear  as  that  presented 
on  the  Marin  Peninsula,  indicates  subsidence  in  late  Quater- 
nary time. 

CHANGES    IN    DRAINAGE. 

The  courses  of  many  of  the  streams  in  these  quadrangles  were 
modified  during  Quaternary  time,  and  it  would  be  interesting 
to  trace  their  history  in  detail,  but  only  a  few  of  the  more 
important  changes  will  be  mentioned. 

The  most  notable  event  in  the  modification  of  the  drainage 
of  the  region  was  the  conversion  of  the  valley  lying  between 
the  Berkeley  Hills  and  the  higher  part  of  the  San  Francisco- 
Marin  Mountain  block  into  an  inlet  of  the  ocean  by  the  sink- 
ing of  the  coast.  The  drainage  from  the  Great  Valley  of 
California  and  from  the  Coast  Ranges  north  and  south  of  San 
Francisco  converged  upon  this  valley  and  flowed  through  a 
short  transverse  canyon  at  the  Golden  Gate.  The  submergence 
admitted  the  sea  to  the  valley  and  so  created  the  great  harbor 
of  San  Francisco  Bay.  By  this  change  the  seat  of  deposition 
of  the  sediments  brought  down  by  the  rivers  from  the  interior 
was  transferred  from  a  delta  outside  of  the  coast  line  to  San 
Francisco  Bay,  which  is  in  consequence  gradually  filling  up. 

Another  remarkable  change  in  the  drainage  of  the  region 
was  the  deflection  to  Alameda  Creek  of  the  waters  that  once 
flowed  from  Livermore  Valley  through  San  Ramon  and 
Ygnacio  valleys  to  Suisun  Bay.  The  divide  between  the  head 
of  San  Ramon  Creek  and  the  h3^drographic  basin  of  Alameda 
Creek,  in  the  countr^'^  east  of  tlie  Concord  quadrangle,  is  situ- 
ated on  the  nearly  level  floor  of  Livermore  Valley,  which  was 
tlie  flood  plain  of  the  much  larger  predecessor  of  San  Ramon 
Creek  before  it  was  beheaded,  somewhat  modified  l)y  recent 
alhiviation.  Tliis  change  explains  the  dispro])ortion  between 
the  large  San  Ramon  Valley  and  the  present  small  creek  that 
flows  through  it. 

San  Pablo  Creek  once  probably  had  a  straight  course  across 
tlie  western  flank  of  Sobrante  Ridge  to  San  Pablo  Bay  in  the 
vicinity  of  Pinole,  just  north  of  the  San  Francisco  quadrangle. 


153 

The  Quaternary  alluvium  at  Pinole,  so  rich  in  the  bones  of 
extinct  animals,  is  doubtless  in  part  the  flood-plain  deposit  of 
the  lower  part  of  San  Pablo  Creek,  carried  there  before  its 
course  was  changed.  The  deflection  of  the  stream's  course 
from  northwest  to  west  near  San  Pablo  is  probably  due  to  its 
capture  by  a  small  stream,  consequent  upon  the  uplift  of  the 
Berkeley  Hills — a  stream  that  eroded  back  rapidly  in  the  soft 
clays  of  the  Orinda  formation. 

Alameda  Creek,  which  passes  through  the  southern  exten- 
sion of  the  Berkeley  Hills  in  a  deep,  narrow  canyon  and  flows 
across  the  alluvial  plain  in  the  southern  part  of  the  Haywards 
quadrangle,  has  an  interesting  relation  to  the  uplift  of  the 
Berkeley  Hills.  It  heads  on  the  slopes  of  Mount  Hamilton 
and  Mount  Diablo  and  drains  broad,  low-lying  valleys  east  of 
the  Berkeley  Hills.  Its  course  across  these  hills  between  Sunol 
and  Niles,  in  the  Pleasanton  quadrangle,  was  evidently  estab- 
lished before  their  uplift,  and  the  stream  has  persisted  across 
the  rising  mountain  block,  cutting  its  rugged  canyon  deeper 
and  deeper  as  the  block  rose.  It  is  an  excellent  example  of 
an  antecedent  stream.  During  most  of  the  time  in  which  it 
was  thus  entrenching  itself  across  the  rising  mountain  block 
Alameda  Creek  was  much  smaller  than  it  is  now,  for  the 
drainage  of  Livermore  Valley,  which  once  flowed  nortli  by 
way  of  San  Ramon  Valley,  in  the  Concord  quadrangle,  and 
is  now  tributary  to  Alameda  Creek,  was  captured  by  that 
creek  late  in  Quaternary  time.  By  this  capture  the  hydro- 
graphic  basin  of  Alameda  Creek  was  more  than  doubled. 

The  convergence  of  a  large  part  of  the  drainage  of  the 
Haywards  and  Concord  quadrangles  at  Castro  Valley,  near 
Haywards,  has  probably  been  in  large  measure  determined  by 
a  structural  sag  in  the  Berkeley  Hills  block,  formed  at  the 
time  of  its  uplift. 

The  drainage  of  the  southwest  slope  of  tlie  Berkeley  Hills 
has  been  described  under  the  heading  "Structure"  and  illus- 
trated in  part  in  figure  4.  The  streams  on  this  slope  origi- 
nall}^  flowed  with  steep  gradients  across  a  zone  of  marked 
deformation  and  faulting  and  were  relatively  straight.  Later 
faulting  along  the  old  fault  zone  apparently  offset  to  a  small 


15i 


extent  some  of  these  streams,  and  other  streams  were  deflected 
along  the  line  of  the  fault  in  a  longitudinal  rift  valle}^  that  was 
formed  in  }3art  directly  by  faulting  but  chiefly  by  excessive 
erosion  of  the  zone  of  crushed  and  mashed  rock  produced  b}^ 
faulting.  The  trench  of  Shepard  and  Diamond  creeks  is  but 
slightly  offset  at  the  rift  valley,  whereas  Viejo  Creek  has  been 
deflected  nearly  a  mile.  Kohler  Creek,  which  was  probably 
once  continuous  with  Hayes  Creek,  is  now  entirely  separated 
from  it  and  flows  along  the  rift  valley  to  Temescal  Creek,  a 
mile  distant.  Other  streams  in  this  vicinity  have  been  more 
or  less  affected  in  a  similar  manner. 

The  remarkably  straight  drainage  line  of  the  San  Andreas 
rift  in  the  San  Mateo  quadrangle  was  begun  probably  in 
Quaternary  time  by  the  creation  of  a  belt  of  soft,  mashed  rock 
on  a  zone  of  recurrent  faulting,  but  the  more  profound  rift  val- 
ley extending  from  Bolinas  Lagoon  to  Tomales  Bay,  with  its 
equally  straight  drainage  lines,  is  an  inheritance  from  a  much 
older  geologic  period,  as  may  be  inferred  from  the  discussion 
of  the  faulting  at  that  place  under  the  heading  "Structure." 

ECONOMIC    GEOLOGY. 

AVAILABLE   RESOURCES. 

The  mineral  resources  of  the  San  Francisco  district  are 
chiefly  building  materials,  such  as  clay,  shale,  limestone,  rock 
suitable  for  crushing,  gravel,  and  sand,  although  small  quanti- 
ties of  gold,  lead,  copper,  manganese,  asbestos,  chromite,  tale, 
magnesite,  and  quicksilver  are  found,  and  manganese  ore  and 
pyrite  have  been  mined  on  a  small  scale.  Volcanic  ash  has 
been  quarried  for  use  as  polishing  material  and  large  quanti- 
ties of  salt  are  extracted  from  the  sea  water.  The  most  valuable 
natural  resource  in  the  region,  however,  is  water. 

WATER. 

Surface  water,  impounded  by  dams  in  the  valleys  of  streams, 
has  been  extensively  used  to  supply  the  cities  on  the  shores 
of  San  Francisco  Bay,  and  the  conversion  of  valleys  into 
reservoirs  for  the  storage  of  rain  water  is  likely  to  be  greatly 


155 

extended  in  the  near  future,  particularly  on  the  east  side  of 
tlie  bay.  Such  utilization  requires  no  special  discussion  here, 
thoui»;h  knowledge  of  geologic  conditions  can  assist  in  deter- 
mining dam  sites  that  are  suitable  as  to  foundation  and  as 
free  as  possible  from  danger  of  disturbance  by  earthquake. 
A  knowledge  of  the  nature  of  the  underlying  formations  and 
of  the  distribution  of  recent  f^iults  may  also  minimize  the 
danger  to  projected  pipe  lines  and  tunnels,  a  fact  that  has 
been  generally  recognized  since  the  earthquake  of  1906. 

The  rain  water  that  is  stored  in  the  pores  and  voids  of  open- 
textured  formations,  such  as  gravels,  sands,  and  sandstones,  is 
also  a  source  of  water  supply.  Sandstone  or  other  porous  rock 
that  is  exposed  directly  to  the  weather  becomes  filled  with 
water  until  it  is  saturated  up  to  a  certain  level  or  plane,  which 
in  tlie  wet  season  may  coincide  locally  with  the  surface  of  the 
ground  but  is  generally  beneath  the  surface.  This  level  is 
known  as  the  ground-water  plane.  The  underground  storage 
in  this  region  is  well  exemplified  in  the  hill  lands,  which  are 
very  generally  composed  of  sandstone  and  hold  water  up  to  a 
large  but  variable  proportion  of  their  volume.  This  ground 
water  tends  to  escape  in  springs  at  the  lower  levels  of  the  hill 
slopes,  and  owing  to  this  leakage  the  ground-water  plane 
becomes  lower  and  lower  during  the  dry  season;  but  as  the 
frictional  resistance  to  flow^  through  the  minute  spaces  in  the 
rocks  greatly  retards  the  escape  of  the  water  the  supply  may 
endure  through  the  dry  season  till  the  leakage  is  made  good 
by  replenishment  during  the  following  rainy  season.  The 
ground  water  in  the  hill  lands  of  this  region  is  most  commonly 
tapped  not  by  wells  but  by  tunnels  driven  into  the  hillsides, 
and  it  is  a  valuable  source  of  local  supply  where  the  consump- 
tion is  small. 

Many  porous  rocks  that  are  well  adapted  for  the  storage  of 
water  are  in  large  pai-t  buried  by  clays  and  other  rehitively 
impervious  strata,  but  in  places  these  pervious  formations  out- 
crop and  surface  water  or  rainfall  at  this  outcropping  edge  sinks 
into  and  fills  the  open-textured  rock.  In  valleys  underlain 
by  such  formations  it  is  generally  possible  to  reach  the  saturated 
strata  by  boring  through  a  mantle  of  impervious  material.     If 


156 


the  porous  outcrop  at  which  the  water  enters  the  rock  is  not- 
ably higher  than  the  top  of  the  bore  hole  the  water  will  flow 
from  the  well,  and  if  the  water  does  not  quite  reach  the  surface, 
owing  to  the  slight  difference  of  level  between  the  point  of 
intake  and  the  valley  floor,  it  may  be  lifted  by  pumps.  Under- 
ground water  supply  of  this  kind,  whether  the  w^ater  overflows 
at  the  surface  or  not,  is  classed  as  artesian. 

The  Bay  of  San  Francisco  is  a  sunken  valley  which  as  it  sank 
gradually  became  filled  with  detritus  washed  in  by  streams 
from  the  surrounding  hills,  the  coarser  and  more  pervious 
deposits  having  been  from  time  to  time  buried  by  finer  and 
less  pervious  silts  and  clays,  part  of  them  fluviatile  or  delta 
deposits  and  part  of  them  marine  clays  or  sands.  The  valley 
in  which  the  bay  lies  is  therefore  an  artesian  basin.  The 
irregular  tongues  of  gravel  that  grow  wider  and  thicker  away 
from  the  mouths  of  the  canyons  are  separated  by  sheets  of  clay 
and  have  been  buried  by  impervious  deposits,  leaving  the 
gravels  exposed  only  near  the  mouths  of  the  canyons,  where 
the  present  streams  flow  over  them.  These  buried  gravel 
tongues  are  thickest  and  most  numerous  in  the  old  deltas  of 
the  larger  streams,  but  it  is  probable  that  in  many  places  the 
gravels  at  the  several  stages  of  the  infilling  coalesce  in  the  axis 
of  the  buried  valley,  so  that  the  water  which  tlie}^  contain  is 
intercommunicating. 

The  largest  stream  that  enters  the  valley  fi"om  the  surround- 
ing hills,  except  of  course  the  streams  in  the  Great  Valley,  is 
Alameda  Creek.  It  has  also  the  largest  buried  delta,  and  this 
delta  is  the  source  of  the  greatest  artesian-water  supply  in  the 
valley.  Alameda  Creek  drains  the  northern  slopes  of  Mount 
Hamilton  and  the  southern  slopes  of  Mount  Diablo,  its  drain- 
age basin  including  about  600  square  miles.  It  emerges  from 
its  gorge  through  the  southward  extension  of  Berkeley  Hills  at 
Niles,  at  an  elevation  of  <S0  feet  above  sea  level.  From  this 
point  the  surfiice  of  its  delta  slopes  gently  downward  in  a  very 
flat  cone,  whose  western  limit  is  the  tidal  marsh  of  the  bay 
shore.  Near  the  apex  of  the  cone,  around  Niles,  the  surface 
is  gravelly,  for  tlie  coarse  material  brought  down  by  the  stream 
is  dropped  where  the  current  is  checked  as  the  stream  emerges 


157 


from  the  narrow  canyon.  The  finer  material,  however,  is 
spread  uniformly  over  the  lower  slopes  of  the  delta  as  a  deposit 
of  clayey  consistency.  This  clay  mantle  seals  the  water  in 
the  sands  and  gravels  of  the  delta,  and  the  upper  edge  of  the 
clay  determines  the  level  at  which  the  artesian  water  under 
natural  conditions  can  stand — about  40  feet  above  sea  level. 

Numerous  wells  sunk  in  this  delta  show  that  an  alternation 
of  gravels,  sands,  and  clays — some  of  the  clays  containing 
marine  fossils  of  living  species — extends  for  several  hundred 
feet  below  sea  level,  but  that  the  sequence  of  this  alternation 
differs  considerably,  even  in  neighboring  wells.  Very  few  of 
the  wells  have  completely  pierced  this  great  buried  delta,  but 
one  well,  on  the  margin  of  the  salt  marsh  west  of  Alvarado, 
reached  bedrock  at  730  feet  below  sea  level.  The  water  sup- 
plying the  wells  is  drawn  from  gravels  which  lie  at  several 
levels  but  all  of  which  are  connected  with  gravels  at  the  apex  of 
the  modern  delta,  unless  possibly  they  have  been  dislocated  by 
faulting  at  the  base  of  the  Berkeley  Hills.  The  fact  that  these 
gravels  extend  down  to  the  present  shore  of  the  bay  and, 
indeed,  beneath  the  bay,  as  is  shown  by  wells  sunk  off  shore, 
indicates  that  the  surface  of  the  delta  has  been  much  steeper 
than  it  is  now,  except  at  periods  of  unusually  active  subsidence, 
during  which  the  sea  flooded  its  surface.  That  the  water  in 
the  delta  gravels  is  in  free  contact  with  the  salt  winter  of  the 
bay  is  shown  by  the  fact  that  when  the  wells  at  Alvarado  are 
left  unpumped  their  water  rises  and  falls  rhythmically  with 
the  flow  and  ebb  of  the  tide,  although  the  movement  shows  a 
lag  of  some  hours  due  to  the  frictional  resistance  of  the  gravels. 
This  fact  suggests  that  the  fresh  water  might  become  contami- 
nated by  the  salt  water,  but  under  noruial  conditions  con- 
tamination is  prevented  by  the  higher  head  of  the  artesian 
water  and  by  its  slow  movement  toward  the  bay  in  consequence 
of  leakage  through  thin  places  in  the  mantle  of  clay.  Most 
such  leaks  are  under  the  bay,  but  some  of  them  may  be  seen  in 
the  marshes  south  of  Alvarado.  Though  this  leakage  suggests 
that  all  the  water  in  the  deltii  might  eventually  drain  down 
nearly  to  sea  level,  that  drainage  is  hindered  by  the  frictional 
resistance  of  the  gravels,  because  of  which  the  artesian  water 


158 


must  have  a  certain  minimum  gradient — probably  about  5  feet 
to  the  mile — before  it  will  flow  at  all.  With  less  gradient  the 
water  would  be  practically  stationary  under  normal  conditions 
and  its  drop  from  wet  season  to  wet  season  would  be  negligible. 
Notwithstanding  the  leakage  to  the  bay  the  gradient  therefore 
maintains  the  level  of  the  water  in  the  delta  at  a  maximum  of 
about  40  feet  above  sea  level,  a  level  which  fortuitously  coin- 
cides with  that  wdiich  would  be  determined  by  the  upper 
margin  of  the  clay  mantle  below  Niles  if  no  leakage  affected 
the  stored  water.  When  wells  were  first  sunk  on  the  lower 
flanks  of  the  delta  the  water  flowed  freely,  but  systematic  draft 
on  this  supply  by  groups  of  wells  pumped  in  series  has  greatly 
steepened  the  gradient  and  lowered  the  water  plane  until  now 
all  the  water  has  to  be  lifted  to  the  surface.  If  this  local 
depression  of  the  water  plane  is  made  too  great  by  excessive 
pumping,  salt  water  will  flow^  in  from  the  bay.  Thus  a  limit 
is  fixed  on  the  extent  to  which  such  wells  may  be  pumped. 
The  gravels  of  the  delta  of  Alameda  Creek  yield  7,000,000 
to  10,000,000  gallons  daily  for  the  water  supply  of  Oakland. 
Manuflicturing  concerns  draw  about  3,000,000  gallons  more, 
and  many  single  wells  also  tap  the  supply. 

The  artesian  condition  above  outlined  probably  exists  also, 
on  a  smaller  scale,  on  all  the  streams  emptying  into  the 
alluvium-filled  parts  of  the  Bay  of  San  Francisco,  and  the 
filling  of  this  sunken  valley  is  in  a  broad  sense  simply  an 
aggregation  of  a  series  of  confluent  or  interdigitating  deltas. 
In  this  sense  the  valley  of  the  bay  constitutes  one  large  arte- 
sian basin.  The  numerous  successful  w^ells  that  have  been 
sunk  in  it,  particularly  around  the  soutliern  end  of  the  bay, 
show  that  the  conditions  favorable  to  the  storage  of  water  are 
nearly  coextensive  with  the  area  of  the  valley  and  that  gravel 
beds  containing  water  lie  at  many  levels. 

The  conditions  at  the  north  end  of  the  valley,  on  the  south 
side  of  Ban  Pablo  Bay,  are  somewhat  similar  to  those  at  the 
south  end.  A  number  of  wells  recently  sunk  near  Ban  Pablo 
have  obtained  a  supply  of  water  from  gravelly  and  sand}^ 
strata  alternating  with  clays,  which  have  an  aggregate  thickness 
of  about  500  feet.     The  situation  of  the  deposits  indicates  that 


159 

their  superficial  parts  are  the  confluent  deUas  of  San  Pablo 
and  Wildcat  creeks,  but  the  wells  doul)tless  pierce  these  depos- 
its and  penetrate  the  underlying,  slightly  inclined  strata  of 
alternating  gravels  and  clays  of  the  Orinda  formation. 

The  artesian  basin  of  San  Francisco  Bay  is  probably  suscep- 
tible of  still  more  extended  exploitation,  and  measures  might 
possibly  be  taken  to  insure  the  permanence  of  this  supply  by 
directing  surface  drainage  into  the  gravel  beds  that  hold  the 
water. 

Another  small  but  similar  artesian  basin,  which,  however, 
has  been  but  Kttle  exploited,  lies  beneath  Ygnacio  Valley,  in 
the  Concord  quadrangle,  and  its  extension  northward  to  Suisun 
Bay. 

BRICK    CLAYS. 

Orinda  formation. — The  Orinda  formation,  a  heterogeneous 
accumulation  of  fluviatile  and  laeustral  deposits  with  local  beds 
of  volcanic  ash,  includes  valuable  beds  of  clay,  the  more 
important  of  which  may  be  briefly  indicated. 

A  large  deposit  lies  near  Glorietta  and  Bryant,  in  the 
Concord  quadrangle.  Midway  between  these  two  places  the 
beds  are  about  75  feet  thick,  but  they  include  a  few  layers  of 
sandy  clay  and  sandstone.  The  clay  outcrops  continuously 
for  nearly  a  mile  south  of  Glorietta,  but  beyond  this  ])oint 
it  is  masked  by  soil.  The  deposit  occurs  also  farther  north, 
just  beyond  Bryant  and  north  of  Orinda.  Throughout  its 
known  extent  the  deposit  averages  probably  40  feet  in  thick- 
ness, but  its  boundaries  are  obscure  and  this  estimate  is  there- 
fore not  exact.  At  Bryant  a  second  set  of  clay  beds  outcrops 
on  the  hillside  west  of  those  above  described.  This  deposit 
aggregates  about  30  feet  in  thickness  but  in  places  includes 
considerable  sand. 

At  the  northwest  end  of  Pleasant  Valley,  near  Lafiiyette,  in 
the  Concord  quadrangle,  there  is  an  exposure  of  sandy  clays, 
which  extend  more  than  a  mile  eastward.  In  its  easternmost 
exposures  the  clay  appears  on  the  south  side  of  the  valley, 
where  its  estimated  thickness  is  about  30  feet. 


160 


South  of  Lafa3^ette,  on  the  road  leading  eastward  from 
Lafayette  to  Moraga  Valley,  there  is  a  deposit  of  clay  of  good 
qiialit}',  which  is  12  to  15  feet  thick  as  exposed  on  the  banks 
of  the  creek.  North  of  the  creek  the  deposit  is  apparently 
thicker. 

On  Las  Trampas  Creek  east  of  Moraga  Valley  about  6  feet 
of  good  clay  is  exposed,  but  the  extent  and  thickness  of  the 
deposit  can  not  be  determined  without  excavatiou.  A  belt  of 
clay  beds  extends  through  Bolinger  Canyon,  but  so  far  as  they 
are  now  exposed  they  appear  to  be  thin  and  to  grade  into 
clayey  sandstones  and  conglomerates. 

Near  Danville  several  beds  of  good  clay  are  exposed  in  the 
creek  banks,  their  total  thickness  being  about  20  feet.  The 
greatest  thickness  of  a  single  bed  is  6  feet.  There  is  also  con- 
siderable sandy  clay  at  the  same  locality. 

Several  other  deposits  of  impure  or  sandy  clay  suitable  for 
brickmaking  occur  in  the  Orinda  area,  and  much  of  the  Orinda 
soil  in  places  where  no  outcrop  of  the  strata  is  apparent  is  so 
clavev  as  to  be  available  for  brickmakina;.  Some  of  these 
clayey  soils  probably  lie  upon  clays  of  good  grade. 

Siesta  formation. — The  Siesta  formation  is  composed  largely 
of  clays,  which  extend  from  the  head  of  Wildcat  Creek,  in 
the  Concord  quadrangle,  southeastward  to  the  end  of  Moraga 
Valley.  The  most  extensive  exposure  of  these  clays  is  in 
Moraga  Valley,  where  the  belt  of  clay  has  a  maximum  width 
of  about  1200  feet.  The  formation  throughout  its  extent  lies 
in  a  synclinal  trough,  and  the  width  just  given  probably 
represents  more  than  double  the  thickness  of  the  clays.  The 
clay  beds  are  flanked  on  tlie  east  by  conglomerate  and  on  the 
west  by  sandstone.  The  clay  beds  outcrop  for  several  hundred 
feet  along  the  strike  but  are  generally  covered  by  a  deep  soil. 
The  clay  is  of  good  c|uality  and  the  supply  is  very  large.  The 
deposit  miglit  be  opened  easily  by  cuts  in  the  hillside. 

Alluvial  and  marsh  clay. — On  the  lower  of  the  delta  slopes 
of  Alameda,  San  Pablo,  and  Walnut  creeks  the  soil  is  very 
clayey  and  beneatli  this  soil  there  are  extensive  flat-lying 
deposits  of  clay.  The  depth  of  these  deposits  is  variable,  and 
at  some  places  beds  of  sand  are  intercalated  between  beds  of 


161 

clay.  Below  these  slopes,  in  the  salt  marshes  around  the  bay, 
there  are  at  many  places  extensive  deposits  of  good  clay,  which 
could  be  exploited  b}^  dredging. 

SHALE   FOR    MAKING    BRICKS   AND    CEMENT. 

The  clay  shale  of  the  Knoxville  formation,  in  its  outcrop 
along  the  Berkeley  Hills,  is  available  for  making  bricks  and 
possibly  also,  in  its  purer  forms,  which  contain  little  or  no 
sand,  for  making  cement.  Most  of  this  shale,  however,  in  so 
far  as  is  indicated  by  its  outcrops,  is  more  or  less  sandy. 

LIMESTONE. 

Deposits  of  limestone  large  and  pure  enough  to  be  of  com- 
mercial value  are  rare  in  the  region  about  the  Bay  of  San 
Francisco.  The  oldest  limestone  (Gavilan  limestone)  is  older 
than  the  quartz  diorite.  Within  the  quadrangles  here  described 
this  rock  occurs  apparently  only  as  inclusions  in  the  quartz 
diorite.  One  of  these  inclusions,  on  the  west  side  of  Pilarcitos 
Canyon,  at  the  south  edge  of  the  San  Mateo  quadrangle,  is 
limited  to  a  few  acres  and  is  surrounded  by  quartz  diorite. 
Similar  areas  occur  at  the  head  of  Tomales  Bay,  on  the  west 
side  of  tlie  valley,  opposite  Poiiit  Beyes  station,  just  beyond 
the  northern  limit  of  the  Tamalpais  quadrangle.  These  lime- 
stones are  coarse  marbles  containing  variable  amounts  of  meta- 
morphic  minerals,  notably  woUastonite  and  graphite.  Most  of 
the  rock  contains  some  magnesia  and  free  silica.  Limestone 
has  been  extensively  quarried  in  the  Gavilan  formation  at 
Santa  Cruz  and  to  a  less  extent  in  the  Gabilan  Range. 

In  the  Franciscan  group  the  foraminiferal  limestone  (the 
Calera  limestone  member  of  the  Cahil  sandstone)  lies  in  an 
intermittent  belt  that  extends  across  the  San  Mateo  quadrangle 
from  Calera.  Valley  southeastward  to  Crystal  Springs  Lake, 
and  a  small  outcrop  of  the  same  limestone  lies  in  the  northwest 
corner  of  the  Tamalpais  quadrangle.  This  limestone  varies 
greatly  in  thickness  and  in  its  content  of  chert,  which  occurs 
in  the  form  of  lenses  and  nodules,  so  that,  although  in  part 
very  pur'^,  it  can  be  quarried  profitably  only  on  a  large  scale 
at  fovorable  localities,  where  its   thickness  is  near  the   maxi- 

San  Francisco— 11 


162 


mum  and  its  content  of  chert  is  at  a  minimum.  The  only 
place  where  this  limestone  is  exploited  in  the  San  Mateo  quad- 
rangle is  on  the  coast  at  Calera  Valley,  where  it  has  been 
quarried  for  some  years.  (See  PI.  III.)  Farther  south,  in 
Permanente  Canyon,  the  same  rock  has  been  regularly  quar- 
ried for  several  years  for  use  in  the  beet-sugar  industry. 

A  few  thin  lentils  of  limestone  occur  in  the  Cretaceous 
rocks,  but  they  have  no  commercial  value.  There  are  also 
many  lentils  of  impure  ferruginous  and  phosphatic  limestone 
in  the   bituminous  shales  and  cherts  of  the  Monterey  group. 

In  the  Orinda  and  Siesta  formations  limestone  lenses  occur 
more  or  less  persistently  at  several  horizons,  and  similar  beds 
of  limestone  are  interstratified  with  the  lavas  of  the  Moraga 
formation.  All  these  lenses  are  of  lacustral  origin  and  most  of 
them  are  siliceous.  The  best  that  can  be  said  of  them  as  to 
their  economic  value  is  that  some  of  them  may  prove  to  be 
of  service  for  local  use. 

More  recent  than  all  of  these  are  certain  travertine  deposits 
that  occur  at  several  localities  and  overlie  several  different 
formations.  Small  patches  lie  on  the  western  slope  of  the 
Berkeley  Hills  north  of  Berkeley.  On  Lime  Ridge,  southeast 
of  Concord,  travertine  occurs  as^a  veneer  in  a  few  places  on  the 
hillsides,  forming  deposits,  which  to  the  inexpert  eye  appear 
much  more  extensive  than  they  really  are.  This  limestone  is 
of  good  quality  and  the  deceptive  appearance  it  presents  as 
to  quantity  has  led  to  the  establishment  of  cement  mills  at 
this  locality. 

CRUSHED    ROCK. 

The  cities  about  the  bay  use  a  large  quantity  of  crushed 
rock  for  both  macadam  and  concrete.  The  choice  of  material 
for  these  uses  is  generally  determined  by  convenience  of  situ- 
ation and  ease  of  transportation.  Probably  the  rock  most 
avaihdjle  and  therefore  most  used  for  macadam  and  for  con- 
crete is  the  hard  sandstone  of  the  Franciscan  group,  which  has 
been  quarried  on  both  sides  of  the  bay  for  many  years,  not- 
al)ly  at  Telegraph  Hill  in  San  Francisco  and  at  Temescal  in 
Oakland.     Quarries  in  this  rock  have    been  opened  recently 


163 

also  on  the  bay  shore  of  Marin  County,  and  it  niiglit  be 
quarried  at  many  other  localities,  but  it  doubtless  varies  in 
quality,  and  considerable  strip])ing  must  generally  be  done  to 
determine  the  character  of  the  rock  below  the  zone  of  weatlier- 
ing.  The  radiolarian  chert  of  the  Franciscan  group  has  also 
been  used  to  some  extent  in  the  city  of  San  Francisco  for 
macadam,  for  which  it  is  very  serviceable  for  light  traffic  if  kept 
thoroughly  watered,  but  most  of  that  which  has  been  quarried 
is  not  well  adapted  for  making  concrete,  because  it  is  inter- 
laminated  with  shale.  Some  beds  of  the  radiolarian  chert, 
however,  are  so  massive  or  thick  bedded  and  include  so  little 
shale  that  they  might  with  advantage  be  used  for  concrete  as 
well  as  macadam.  The  foraminiferal  Calera  limestone,  which 
is  quarried  in  a  large  way  at  Calera  Valley,  is  used  chiefly  as 
crushed  rock.  Occasionally  the  spheroidal  basalt  and  the 
metamorphic  schist  of  the  Franciscan  group  have  been  utilized, 
but  very  few  quarries  have  been  maintained  for  any  consider- 
able time  in  these  rocks.  The  Leona  rhyolite,  which  outcrops 
in  a  nearly  continuous  belt  from  Berkeley  to  Hay  wards  along 
the  west  front  of  the  Berkeley  Hills,  has  for  many  years  been 
quarried,  particularly  at  Laundry  Farm,  back  of  Oakland,  and 
has  yielded  a  great  quantity  of  macadam  for  the  Oakland 
streets. 

The  chert  and  shale  of  the  Monterey  group  have  been  used  to 
some  extent  on  roads  in  Contra  Costa  County  and  make  a 
road  bed  that  is  well  adapted  to  light  traffic.  This  material 
contains  little  or  no  clay,  packs  well,  and  forms  a  road  that  is 
neither  dusty  in  summer  nor  muddy  in  wet  weather. 

The  basalts  and  some  of  the  andesites  of  the  Berkeley  group 
would  afford  excellent  material  for  both  macadam  and  concrete 
but  have  not  yet  been  much  used.  Another  abundant  source 
of  crushed  rock  that  has  not  been  touched  is  the  quartz 
diorite  ("granite")  of  Montara  Mountain  and  the  Point  Reyes 
Peninsula. 

GRAVEL    AND    SAND. 

Gravel  and  gravelly  sand  are  much  sought  by  contractoi'S  for 
making  concrete  in  the  cities  about  the  Bay  of  San  Francisco, 


164 


but  the  supply  is  small,  so  that  a  considerable  quantity  is 
brought  in  from  deposits  beyond  the  area  here  described. 

Tlie  largest  local  supply  of  gravel  lies  at  the  apex  of  the 
delta  of  Alameda  Creek,  at  Niles,  just  east  of  the  Hay  wards 
quadrangle.  Another  deposit  is  on  the  delta  of  Walnut  Creek 
a  short  distance  south  of  Concord,  where  pits  show  a  few  acres 
of  clean  fine  gravel  from  2  to  10  feet  thick.  The  region  is 
soil  covered,  and  much  more  gravel  may  lie  below  the  soil. 

The  wind-blown  sand  in  San  Francisco  is  used  to  some  extent, 
but  it  is  not  sharp  enough  and  is  generally  too  fine  for  many 
of  the  purposes  for  which  sand  is  sought.  There  is  an 
inexhaustible  supply  of  sand  on  the  ocean  beach  for  some  miles 
south  of  the  Golden  Gate,  but  few  attempts  have  been  made  to 
use  it.  The  better  grades  of  sharp  beach  sand  are  brought 
from  the  coast  of  Monterey  County. 

Some  of  the  tidal  flats  on  the  east  side  of  the  bay  afford 
sand  that  serves  to  supply  in  part  the  demand  in  Oakland  and 
Berkeley. 

ABRASIVE. 

The  volcanic  ash  in  the  Merced  formation  south  of  San 
Francisco  has  been  quarried  and  used  as  a  polisher.  The  ash 
is  made  up  of  particles  of  fi'othy  glass  and  is  free  from  quartz 
grains,  so  that  it  is  very  uniform  in  quality  and  is  well  adapted 
for  polishing  wood. 

GREENSAND. 

Some  portions  of  the  Martinez  formation  consist  largely  of 
greensand,  which  may  possibly  be  used  as  a  source  of  potash 
for  incorporation  in  artificial  fertilizers. 

OIL. 

Although  the  sandstones  of  tlie  Monterey  group  are  oil 
bearing  in  some  parts  of  California,  attempts  to  find  oil 
in  the  region  here  described  have  been  unsuccessful.  Sev- 
eral wells  have  been  sunk  in  the  region  east  of  San  Pablo 
Valley,  in  the  Concord  quadrangle,  but  all  of  them  proved 
to  be  dry.     A  small  seepage  of  oil  was  obtained    in  a  shaft 


165 

sunk  many  years  ago  at  the  north  end  of  San  Pablo  Ridge 
in  the  San  Francisco  quadrangle.  The  most  promising 
seepages  are  those  tliat  escape  from  the  bituminous  shales 
of  the  Monterey  group  on  the  Point  Reyes  Peninsula  near 
Duxbury  Point. 

SALT. 

The  winning  of  salt  from  sea  water  by  evaporation  on  the 
marshes  on  both  sides  of  the  Bay  of  San  Francisco  has  been  a 
local  industry  for  many  years. 

PYRITE. 

The  Leona  rhyolite  contains  deposits  of  pyrite  which  liave 
been  mined  for  several  years  for  making  sulphuric  acid.  The 
mines  are  at  Leona  Heights,  near  Laundry  Farm,  southeast  of 
Oakland.  The  pyrite  occurs  as  rather  massive  bodies  of  irreg- 
ular shape,  having  in  some  parts  sharply  defined  and  in  others 
vaguely  defined  boundaries.  The  best-known  deposit,  at  the 
"Leona  Heights  new  mine,"  has  been  studied  particularly  by 
A.  R.  Whitman,  from  whose  unpublished  notes  the  following 
details  are  taken. 

The  ore  has  practically  no  gangue  but  contains  here  and  there 
small  horses  of  country  rock.  It  consists  of  pyrite  mixed  with 
chalcopyrite  and  pyrrhotite  and  a  little  silica.  It  also  contains 
very  small  quantities  of  gold  and  silver.  Some  native  copper 
has  also  been  found  in  older  workings,  and  the  mine  water  is 
charged  with  copper  sulphate. 

The  deposits  are  metasomatic  replacements  of  the  rhyo- 
lite, formed  at  a  time  when  the  ground  water  stood  higher 
with  reference  to  the  ore  bodies  than  it  does  at  present. 
The  retreat  of  the  ground  water  to  lower  levels,  owing  to 
the  deformation  and  dissection  of  the  region,  has  led  to  the 
partial  oxidation  of  some  of  these  deposits  and  the  conse- 
quent production  of  gossans  of  hematite  and  limonite.  The 
oxidation  of  the  sulphides  has  given  rise  to  other  second- 
ary minerals,  of  which  melanterite,  pisanite,  boothite,  chal- 
canthite,  copiapite,  epsomite,  and  alunogen  have  been  described 
by  Schaller. 


166 


It  is  worthy  of  note  that  these  pyritic  deposits  are  confiQed 
to  the  rhyolite  and  that  the  unoxidized  rock  contains  minute 
crystals  of  pyrite,  which  are  rather  thickly  but  sporadically 
scattered  through  it,  and  also  that  the  rhyolite  is  the  only  rock 
in  the  district  containing  disseminated  pja-ite  crystals.  A 
sample  of  the  unoxidized  rhyolite  charged  with  pyrite  was 
assayed  and  found  to  contain  gold  to  the  value  of  16  cents  to 
the  ton. 

It  is  probable  not  only  that  the  ore  replaced  the  rhyolite  but 
that  the  materials  that  were  originally  disseminated  through 
the  rock  have  been  concentrated  during  the  process  of  replace- 
ment. The  rhyolite  has  been  decomposed  by  sulphuric  acid 
produced  by  the  oxidation  of  the  disseminated  pyrite  near  the 
surface  of  the  rhyolite.  The  formation  was  originally  much 
thicker  than  it  is  now,  and  as  the  surface  was  gradually  lowered 
by  erosion  the  process  has  been  continuous.  Theoretically, 
one  result  of  the  process,  besides  the  chemical  attack  on  the 
silicates,  should  be  that,  as  the  available  oxygen  became 
exhausted  near  the  surface,  the  ferrous  sulphate  would  descend 
to  lower  parts  of  the  formation  and  be  reduced  in  the  ground 
water  to  sulphide.  The  course  of  these  descending  solutions 
and  the  sites  favorable  to  the  precipitation  of  the  sulphides  on 
reduction  would  be  determined  by  the  rock  structure.  The 
deposits  are  very  similar  in  their  essential  features  to  larger 
and  more  cupriferous  deposits  in  similar  rocks  elsewhere,  many 
of  which  have  been  explained  as  the  result  of  ascending  mag- 
ma tic  water.  The  genesis  of  the  deposits  at  Leona  Heights, 
however,  is  apparently  simpler;  they  may  be  ascribed  to  the 
action  of  ordinary  meteoric  waters.  Other  similar  deposits  of 
pyrite  will  probably  be  found  in  the  bod}^  of  the  Leona 
rhyolite. 

QUICKSILVER. 

On  the  west  slope  of  the  Berkeley  Hills,  north  of  Berkeley, 
cinnabar  has  been  found  in  a  silieified  rhyolite  fault  breccia 
tliat  outcrops  as  a  prominent  knob.  Assay  of  samples  taken 
from  the  outcrop  showed  that  the  rock  contains  0.5  per  cent  of 
quicksilver.     On  the  slopes  below  this,   however,  loose  frag- 


167 

ments  of  inueli  riclier  ore  have  been  found.     No  attempt  has 
been  made  to  prospect  the  deposit. 

MANGANESE. 

Manganese  ore  in  the  form  of  psilomelane  occurs  at  many 
places  in  the  Coast  Ranges,  where  it  is  associated  with  the 
radiohirian  chert  of  the  Franciscan  group,  and  at  some  of  these 
places  the  ore  is  sufficiently  pure  and  abundant  to  warrant 
mining  on  a  small  scale.  The  only  occurrences  worthy  of 
mention  within  tlie  territory  described  in  this  folio  are  on  Red 
Rock  Island,  in  the  Bay  of  San  Francisco,  and  near  Fort 
Baker,  on  the  north  side  of  the  Golden  Gate,  in  the  San 
Francisco  quadrangle.  At  the  locality  first  named  the  ore 
occurs  on  the  southwest  side  of  the  island  as  an  integral  part 
of  the  radiolarian  chert.  The  chert  here  has  a  west-northwest 
strike  and  a  nearly  vertical  dip,  and  consists  of  the  usual  rhyth- 
mical alternation  of  thin  beds  of  liard,  flinty  to  quartzose 
chert  and  partings  of  shale,  the  whole  having  a  reddish  color. 
Locally,  however,  the  chert  layers  are  sharply  bent  and  con- 
torted, and  back  from  the  shore  at  this  place  the  prevailing  dip 
is  southerly,  at  high  angles.  A  belt  parallel  to  the  strike  of  the 
chert  includes  interstratified  layers  of  psilomelane,  which,  by 
reason  of  their  black  color,  present  a  bold  contrast  to  the 
adjoining  rocks.  Most  of  the  layers  are  about  one-fourth  to 
one-half  inch  thick  and  replace  locally  the  usual  shale  parting, 
but  in  places  the  psilomelane  is  nuicli  thicker.  Some  of  the 
layers  of  the  chert  adjoining  these  layers  of  psilomelane  are 
also  so  charged  with  that  mineral  as  to  be  quite  black,  though 
they  are  still  hard  and  siliceous.  The  psilomelane  also  occurs 
in  minute  particles  in  the  shale  between  the  chert  beds,  making 
it  black.  The  borders  of  the  belt  in  which  these  layers  of  psi- 
lomelane occur  are  not  sharply  defined,  but  the  mineral  has  been 
mined  in  open  cuts  from  2  to  6  feet  wide.  The  ore  appears  to 
be  essentially  a  primary  deposit,  contemporaneous  with  the 
deposition  of  the  silica  that  formed  the  cherts,  although  doubt- 
less some  of  the  psilomelane  has  since  migrated  into  the 
adjoining  beds.  It  is  probably  related  in  its  genesis  to  the 
concretionary  manganese  nodules  now  accumulating  on  the  sea 


168 


floor  in  association  with  siliceous  oozes,  as  shown  by  deep-sea 
dredging. 

Near  Fort  Baker,  in  a  road  cut,  the  manganese  ore  is  well 
exposed  as  a  stratified  deposit  of  hard,  clean  psilomelane  about 
18  inches  thick,  grading  off  in  its  upper  part  into  a  lean  ore 
consisting  of  chert  and  shale  highly  charged  with  the  black 
manganese  mineral.  There  is  no  definite  boundary  between 
this  lean  ore  and  the  normal  radiolarian  cherts,  for  the  propor- 
tion of  psilomelane  simply  decreases  till  it  ceases  to  color  the 
rock.  The  thickness  of  the  ore-impregnated  cherts  above  the 
layer  of  psilomelane  is  about  12  feet.  This  body  of  ore  lies 
within  a  few  feet  of  an  intrusive  contact  of  ellipsoidal  basalt 
with  the  cherts,  the  contact  plane  being  parallel  to  the  bedding, 
and  may  be  traced  for  90  feet  on  the  outcrop  of  the  formation, 
which  dips  about  40°  SW.  In  several  samples  assayed  by 
D.  C.  Billick  the  psilomelane  contains  gold  not  exceeding  40 
cents  to  the  ton. 

Analyses  of  psilomelane  from  California. 


1 

3 

3 

4 

5 

MnO 

61.66 

8.75 

54.98 
7.88 
4.66 

\    3.74 

s 

56.78 

8.45 

8.31 

\    1.80 

69.69 

15.88 

Trace. 

4.16 

67  29 

0 

12  40 

BaO 

6.38 

FesOs 

3.58 
1.43 

Trace. 

AlgOa 

.22 

MgO . 

1.10 

3.41 

Trace. 

3.99 

K,0 

2.59 
15.01 

4.88 

SiOs 

11.68 
13.13 

16.94 
7.26 

Trace. 

H^O 

7.88 

Specific  gravity    .  -     .     .     . 

97.90 
3.76 

96.07 
3.31 

99.51 
3.72 

98.23 
3.72 

94.17 
3.5 

1.  Red  Rock  Island,  Bay  of  San  Francisco. 

2.  Howell  Mountain,  Napa  County. 

3.  Penitencia  Creek,  Santa  Clara  (^ounty. 

4.  Bernal  Heights,  San  Francisco. 

5.  Corral  Hollow,  Alameda  County. 


The  chemical  character  of  the  manganese  ore  at  Red  Rock 
Island  is  shown  in  partial  analysis  1  of  the  table,  which 
includes  also,  for  comparison,   analyses  of  other  similar  ores 


169 


from  neigliboriDg  parts  of  the  Coast  Ranges.  All  the  analyses 
were  made  by  Norman  E.  Smith  in  the  laboratory  of  the 
University  of  California. 

The  shale  partings  north  of  the  manganese  ore  outcrop 
on  Red  Rock  Island  are  abnormally  thick,  generally  meas- 
uring from  half  an  inch  to  2  inches,  and  at  some  places 
the  shale  is  1  to  2  feet  thick.  It  has  a  soft,  earthy  consis- 
tency and  a  shaly  structure  and  is  prevailingly  red,  though 
the  color  grades  locally  into  yellow.  This  shale  was  at  one 
time  mined  as  mineral  paint.  Its  composition  is  shown  in 
the  following  analysis,  made  by  Irving  Miller  at  the  Univer- 
sity of  California. 

Analysis  of  shale  from  Red  Rock  Island,  San  Francisco  Bay,  Cal. 

SiO, 50.58 

TiOg .55 

AI2O3 14.35 

PegOs ^ 15.64 

FeO .65 

MnO .36 

CaO 1.77 

MgO 3.08 

Na^O .70 

K2O 3.84 

H,0(at  no°C.)-  — -- 3.30 

Ignition 5.19 

Soluble .38 

100.  39 
LEAD. 

Where  Euclid  Avenue  crosses  Cordonices  Creek  in  the  city 
of  Berkeley  lumps  of  coarse  crystalline  galena  have  been  found, 
some  weighing  over  100  pounds.  The  source  of  these  lumps 
is  not  known,  but  they  may  have  been  derived  from  deposits 
in  the  Franciscan  group. 

GOLD. 

Gold  occurs  in  the  black  sands  on  the  ocean  beach  south  of 
San  Francisco  and  many  attempts  have  been  made  to  recover 
it,  but  the  quantity  of  black  sand  is  too  small  to  warrant  the 
construction  of  a  washing  or  leaching  plant. 

In  the  shales  and  metamorphic  schists  of  the  \vest  flank  of 
the  Berkeley  Hills    there  are   numerous  stringers  and   small 


170 


veins  of  quartz,  some  of  which  cany  gold.  Fragments  of 
quartz  containing  visible  free  gold  have  been  found  on  the 
slopes  nortli  of  Berkeley. 

COPPER. 

A  little  chalcopyrite  has  been  found  in  a  small  vein  in 
sandstone  of  the  Frauciscan  group  in  the  Tamalpais  quad- 
rangle on  the  east  side  of  the  rift  valley  between  Tomales  and 
Bolinas  bays.  The  chalcopyrite  and  native  copper  which 
occur  in  the  pyritic  deposits  of  Leona  Heights  liave  already 
been  mentioned. 


ASBESTOS,    CHROMITE,    TALC,    AND    MAGNESITE. 

The  serpentine  rocks,  which  occur  in  all  of  the  quadrangles, 
contain  asbestos,  chromite,  talc,  and  magnesite,  but,  so  far 
as  known,  not  in  sufficient  abundance  to  be  of  economic 
importance. 

MINERALS. 

The  following  minerals  occur  in  the  San  Francisco,  Tamal- 
pais, San  Mateo,  Concord,  and  Haywards  quadrangles.  The 
list  has  been  checked  by  Prof  A.  S.  Eakle. 


Native  sulphur. 

Gold. 

Native  copper. 

Native  mercury. 

Galena. 

Cinnabar. 

Pyrrhotite 

Chalcopyrite. 

Pyrite. 

Halite. 

Quartz. 

Chalcedony. 

Jasper. 

Opal. 

Cuprite. 

Hematite. 

Ilmenite. 

Majrnetite. 

Chromite. 


Pyrolusite. 

Limonite. 

Brucite. 

Psilomelane. 

Calcite. 

Magnesite. 

Ankerite. 

Aragonite. 

Malachite. 

H  ydromagnesite. 

Albite. 

Labradorite. 

Enstatite. 

H  ypersthene. 

Diopside. 

Diallage. 

Peetolite. 

Tremolite. 

Actinolite. 


Glaucophane. 

Crossite. 

Garnet. 

Olivine. 

Zircon. 

Datolite. 

Epidote. 

Lawsonite. 

Gyrolite. 

Apophyllite. 

Anal  cite. 

Natrolite. 

Muscovite. 

Blotite. 

Margarite. 

Prochlorite. 

Glauconite. 

Serpentine. 

Chrysotile. 


Talc. 

Titan  ite. 

Apatite. 

Vivianite. 

Gypsum. 

Epsom  ite. 

Melanterite. 

Pisanite. 

Boothite. 

Chalcanthite. 

Halotrichite. 

Alunogen. 

Copiapite. 

Lignite. 

Petroleum. 

Bitumen. 


171 


EARTHQUAKES    AND    CONSTRUCTION. 

The  well-known  susceptibility  of  the  region  about  San 
Francisco  Bay  to  earthquakes  naturally  raises  the  question 
how,  in  the  light  of  geologic  knowledge,  loss  of  life  and 
property  due  to  violent  shocks  may  be  guarded  against  or 
minimized.  In  considering  this  question  it  should  be  noted, 
first,  that  more  than  99  per  cent  of  the  earthquakes  that  affect 
the  region  are  harmless.  They  are  tremors  of  the  earth's 
crust  due  to  the  adjustments  of  minor  stresses  in  the  rocks  far 
below  the  surfiee.  In  regions  where  such  tremors  are  frequent, 
however,  as  in  the  region  about  San  Francisco  Bay,  violent 
and  destructive  shocks  occur  also,  tliough  at  comparatively 
long  intervals,  and  it  is  to  these  greater  shocks,  of  course,  that 
attention  is  particularly  directed. 

Violent  shocks  are  due  to  the  sudden  adjustment  of  major 
stresses  in  the  earth's  crust.  These  stresses  accumulate  through 
a  long  period  of  time  and  involve  a  slow  torsional  deformation 
of  tlie  portion  of  the  crust  immediately  affected.  The  ultimate 
cause  of  these  stresses  can  not  yet  be  positively  stated,  although 
there  has  been  much  speculation  upon  that  question.  In  the 
firm  rocks  of  the  earth's  crust  the  torsional  deformation  is 
chiefly  elastic  and  is  sooner  or  later  relieved  by  rupture.  AVhen 
ruptui'e  occurs  there  is  an  elastic  rebound,"  the  part  of  the 
earth's  crust  on  one  side  of  the  rupture  plane  or  fault  moving 
suddenly  forward  in  the  direction  of  tlie  former  slow  creep  due 
to  stress  and  the  part  on  the  other  side  springing  as  suddenly 
back  in  the  opposite  direction.  The  quick  dislocation  of  the 
rocks  which  is  thus  brouglit  about  causes  a  jar  whicii  is  propa- 
gated as  a  vibratory  movement  through  the  earth,  particularly 
around  the  outer  shell  or  crust  of  the  earth,  giving  rise  to  the 
commotion  known  as  an  earthquake.  When  such  a  major 
stress  in  the  rocks  has  been  thus  relieved  by  faulting  many 
years  may  elapse  befbre  it  can  reaccumulate  sufficiently  to 
overcome  friction  and  cementation  on  the  old  plane  of  rupture 
and  so  produce   another  slip  and  another  large  earthquake, 

"See  Reid,  H.  F.,  The  elastic  rebound  tlieory  of  earthquakes:  California 
Univ.  Dept.  Gt^ology  Bull.,  vol.  6,  No.  19,  1911. 


172 


though  maDy  minor  adjustments  and  consequent  small  shocks 
may  occur  in  the  affected  zone.  It  thus  happens  that  cata- 
strophic earthquakes  due  to  movement  on  any  single  fault  or 
fault  zone  generally  occur  only  at  long  intervals. 

Now  in  any  effort  to  mitigate  the  recurrent  evils  of  cata- 
strophic earthquakes  in  a  region  of  dense  population,  like  the 
one  here  considered,  the  first  requirement  is  a  knowledge  of 
the  location  and  character  of  all  the  faults  which  traverse  it  or 
the  country  adjacent  to  it.  A  long  step  has  been  taken  in  this 
direction  in  the  geologic  mapping  of  the  five  quadrangles 
described  in  the  San  Francisco  folio  and  of  the  adjacent  quad- 
rangle to  the  south,  described  in  the  Santa  Cruz  folio  by  other 
geologists.  In  the  study  of  these  quadrangles  many  faults 
have  come  to  light  and  are  indicated  on  the  maps.  Future, 
more  detailed  studies  will  doubtless  reveal  others  and  w'ill  also 
add  to  our  knowledge  of  the  direction  of  movement,  the 
amount  of  displacement,  and  the  geologic  age  of  the  faults 
that  are  now  known. 

Next,  among  the  many  faults  thus  recognized  it  is  necessary 
to  discriminate  between  those  upon  which  there  is  no  probabil- 
ity of  future  movement  and  which  are  therefore  harmless  and 
those  which  lie  in  zones  of  active  stress  and  which  are  therefore 
dangerous.  Of  the  many  faults  discovered  in  the  region  of 
San  Francisco  Bay  only  two  are  certainly  known  to  be  zones 
of  active  stress.  These  are  the  San  Andreas  fault  and  the 
Haywards  fault,  each  of  which  is  a  recDrd  of  a  catastrophic 
earthquake.  Other  zones  of  active  stress  may  yet  be  dis- 
covered, but  most  of  the  faults  are  the  expression  of  energies 
that  have  been  long  spent  and  are  not  in  any  sense  a  menace. 
It  is,  moreover,  barely  possible  that  the  stresses  in  the  San 
Andreas  fault  zone  have  been  completely  and  permanently 
relieved  by  the  fault  movement  of  1906.  We  have  only  one 
way  of  determining  this,  and  that  is  by.  repeated  geodetic  sur- 
veys to  discover  whether  or  not  the  latitude  and  longitude  or 
idtitude  of  selected  points  in  the  zone  are  changing  under 
stress  creep.  Such  surveys  would  also  afford  the  most  certain 
way  in  general  of  discriminating  between  zones  of  active  stress 
and  zones  in  which  stress  has  been  completely  relieved.     In 


173 


their  absence  we  can  judge  of  the  probable  danger  of  a  fault 
only  by  the  evidence  of  the  recency  of  its  last  movement,  and 
this  evidence  may  be  historic,  or  geomorphic,  or  geologic.  If 
we  have  positive  evidence  of  recent  movement — evidence  of 
any  of  these  three  kinds — then  all  structures  such  as  roads, 
bridges,  aqueducts,  pipes,  and  tunnels,  which  cross  the  fault, 
are  in  danger  of  destruction,  and  every  effort  should  be  made 
in  their  design  not  only  to  minimize  the  destructive  effect  but 
also  to  supply  auxiliary  structures  to  tide  over  a  period  of 
repairs.  This  principle  has  been  recognized,  for  example,  in 
the  construction  of  the  aqueduct  from  Owens  River  to  liOS 
Angeles.  This  important  structure  parallels  the  Inyo  fault, 
on  which  movement  occurred  in  1872,  and  crosses  the  San 
Andreas  fault,  on  which  movement  occurred  in  1857.  The 
engineers  who  designed  the  structure  have  recognized  the 
danger  inherent  in  its  situation  by  providing  reservoirs  on  the 
city  side  of  the  danger  zone,  which  will  insure  a  supply  of 
water  pending  repairs  to  any  break  that  might  be  caused  by  a 
recurrence  of  faulting.  Even  where  there  is  no  reason  to 
suspect  recent  movement  on  faidt  lines  engineers  should  avoid 
them  as  far  as  possible  in  locating  important  works.  This 
principle  has  been  recognized  by  the  engineers  of  the  People's 
AVater  Co.  in  locating  many  dams  and  tunnels  in  the  Concord 
and  San  Francisco  quadrangles  and  by  the  engineers  of  the 
Spring  Valley  Water  Co.  in  locating  the  foundation  for  the 
new  Calaveras  dam,  south  of  Sunol. 

Besides  the  dangers  that  arise  from  the  rupture  and  displace- 
ment of  the  ground  and  that  may  either  be  avoided  by  wisely 
selecting  the  locations  for  important  sti'uctures  or  be  mini- 
mized by  providing  auxiliary  structures  and  facilities  for  speedy 
repairs,  there  are  other  more  general  dangers  due  to  the  vibra- 
tion of  the  ground,  concerning  which  a  word  of  caution  may  be 
of  service.  The  principle  to  be  observed  by  those  who  may 
design  and  locate  large  buildings  or  works  in  this  region  is 
that  all  structures  which  rest  on  solid  rock  are  very  much  safer 
than  those  which  rest  on  loose,  unconsolidated  ground,  whether 
the  ground  is  natural  or  artificial,  and  that  loose  ground  satur- 
ated with  water  is  the  most  dangerous  of  all.     This  principle 


174 


lias  been  recognized  by  the  enoineers  of  tlie  city  of  San  Fran- 
cisco in  the  construction  of  the  new  salt-water  fire-protection 
system,  all  the  mains  being  laid  so  far  as  possible  on  firm  rock. 
It  has  not  been  so  well  recognized,  how^ever,  in  the  rebnild- 
ing  of  lower  San  Francisco,  wdiere,  on  the  made  ground, 
many  buildings  have  been  erected  on  foundations  which,  so 
far  as  earthquakes  are  considered,  are  most  unstable.  The 
deep  piHng  which  is  commonly  employed  in  constructing  large 
foundations  and  the  use  of  steel  frames  in  modern  con- 
struction, however,  very  largely  offset  the  danger  inherent  in 
the  foundations. 

Another  principle  to  be  observed  in  any  region  subject  to 
severe  earthquake  shocks  is  simplicity  and  unity  of  structure. 
Two  structural  types  combined  in  the  same  building  and  not 
intimately  and  strongly  tied  together  vibrate  with  different 
periods  and  mutually  tend  to  destroy  each  other.  This  prin- 
ciple is  perhaps  not  sufficiently  recognized  by  all  constructors 
who  veneer  steel  frames  with  masonry.  The  masonry  should 
invariably  be  thoroughly  bonded  to  the  steel.  The  failure  to 
recognize  this  principle  causes  the  overthrow  of  brick  chimneys 
in  ordinary  frame  buildings.  The  house  and  the  chimney 
swing  with  different  periods  under  the  impulse  imparted  by 
the  movement  of  the  ground  and  the  chimney  is  broken  off*, 
usually  at  the  roof  line.  For  residences  the  most  earthquake- 
proof  house  is  a  frame  structure  that  is  firmly  tied  to  a  con- 
crete foundation  and  that  has  no  brick  chimney.  In  general 
all  buildings  erected  in  a  country  subject  to  severe  earthquakes 
should  be  made  stronger  than  buildings  erected  elsewhere, 
and  the  best  provision  against  partial  destruction  is  a  large 
margin  of  safety  in  strength. 

Finally  it  may  be  remembered  tliat,  although  the  coast  of 
California  has  never  suffered  in  historic  times  from  a  sea-wave 
generated  by  a  fault  on  the  sea  floor,  such  an  event  is  not 
beyond  the  range  of  possibility.  The  only  way  to  guard 
against  this  danger  is  to  avoid  phicing  buildings  where  large 
numbers  of  people  congregate  close  to  sea  level. 


175 

LITERATURE. 

The  following  is  a  chronologic  list  of  the  more  important 
papers  dealing  with  the  geology  of  the  region  mapped  in  the 
present  folio : 

Belchkr,  Lieutenant  Edward,  and  CoLTiiE,  A.,  Geological  notes  [selected 
by  Prof.  Buckland]:  Zoology  of  Captain  Beechey's  voyage,  London,  1839. 

Dana,  J.  D.,  Geology:  Rept.  U.  S.  Exploring  Expedition,  Charles  Wilkes, 
U.  S.  N.,  commanding,  vol.  10,  Philadelphia,  1849. 

Tyson,  P.  T. ,  Report  upon  the  geology  of  California:  Senate  Ex.  Doc. 
No.  47,  31st  Cong.,  1st  sess.,  Washington,  1850. 

Tyson,  P.  T.,  Geology  and  industrial  resources  of  California,  Baltimore, 
1851. 

Trask,  J.  B.,  Report  on  the  geology  of  the  Coast  Mountains  [etc.]:  Journal 
of  the  Senate,  5th  sess.,  Doc.  No.  9,  Sacramento,  1854. 

BiiAKE,  W.  P.,  On  the  characters  and  probable  geological  age  of  the  sand- 
stone formation  of  San  Francisco:  Am.  Assoc.  Adv.  Sci.  Proc.  for  1855, 
Salem,  1856. 

Blake,  W.  P.,  Observations  on  the  physical  geography  and  geology  of  the 
coast  of  California  from  Bodega  Bay  to  San  Diego:  U.  S.  Coast  Survey 
Rept.  for  1855,  App.  65,  Washington,  1856. 

Bachk,  a.  D.,  Upon  the  existence  of  a  deposit  of  red  sand  inside  of  the  bar 
of  San  Francisco  Bay,  California:  U.  S.  Coast  Survey  Rept.  for  1855, 
App.  20,  Washington,  1856. 

Bl/AKE,  W.  P.,  Geological  report:  Reports  of  explorations  and  surveys 
[etc.]  for  a  railroad  from  the  Mississippi  River  to  the  Pacific  Ocean, 
vol.  5:  Senate  Doc.  No.  78,  33d  Cong.,  2d  .sess.,  Washington,  1856. 

Antisell,  Thomas,  Geological  report:  Reports  of  explorations  and  surveys 
[etc.]  for  a  railroad  from  the  Mississippi  River  to  the  Pacific  Ocean, 
vol.  7:  Senate  Doc.  No.  78,  33d  Cong.,  2d  sess.,  Washington,  1857 

Newberry,  J.  S.,  Geological  i*eport:  Repoi-ts  of  explorations  and  surveys 
[etc.]  for  a  railroad  from  the  Mississippi  River  to  the  Pacific  Ocean, 
vol.  6:  Senate  Doc.  No.  78,  33d  Cong.,  2d  sess.,  Washington,  1857. 

R^MOXD,  A.,  Description  of  lour  new  species  of  Echinodermata  from  the 
Tertiary  of  Contra  Costa  County:  California  Acad.  Sci.  Proc,  vol.  3, 
San  Francisco,  1863. 

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Bowman,  A.,  On  coast,  surface,  and  scenic  geology:  California  Acad.  Nat. 
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Becker,  G-.  F.,  Notes  on  the  stratigraphy  of  Calif oi-nia:  U.  S.  Geol.  Sur- 
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Becker,  G.  F. ,  Geology  of  the  quicksilver  deposits  of  the  Pacific  slope: 
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Le  Coxte,  Joseph,  Tertiary  and  jsost -Tertiary  changes  of  the  Atlantic 
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Turner,  H.  W.,  The  geology  of  Mount  Diablo,  California:  Geol.  Soc. 
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Fairbanks,  H.  W.,  The  pre-Cretaceous  age  of  the  metamorphic  rocks  of 
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Fairbanks,  H.  W.,  Notes  on  a  further  study  of  the  pre-Cretaceous  rocks 
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Palache,  Charles,  On  a  rock  froia  the  vicinity  of  Berkeley  containing  a 
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Ransome,  F.  L.,  The  geology  of  Angel  Island:  California  Univ.  Dept. 
Geology  Bull.,  vol.  1,  No.  7,  Berkeley,  1894. 

Lawson,  a.  C,  The  geomorphogeny  of  the  coast  of  northern  California: 
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Lawson,  A.  C,  Sketch  of  the  geology  of  the  San  Fi-ancisco  Peninsula: 
U.  S.  Geol.  Survey  Fifteenth  Ann.  Rept.,  AVashington,  1895. 

Fairbanks,  H.  W.,  Review  of  our  knowledge  of  the  geology  of  the  Cali- 
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Ransome,  F.  L.,  On  law.sonite,  a  new  rot-k-toniiinj^C  mineral  Ironi  the 
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Fairbanks,  H.  W.,  The  stratigraphy  of  the  California  Coast  Ranges: 
Jour.  Geology,  vol.  3,  Chicago,  1895. 

Merriam,  J.  C,  Sigmogomphius  lecontei,  a  new  castoroid  rodent  from  the 
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Chapman,  F.,  On  some  Pliocene  Ostraeoda  from  near  Berkeley:  Cali- 
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Stanton,  T.  W.,  The  faunal  relations  of  the  Eocene  and  Upper  Cretaceous 
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pt.  1,  Washington,  1896. 

Fairbanks,  H.  W.,  The  geology  of  the  San  Francisco  Peninsula:  Jour. 
Geology,  vol.  5,  Chicago,  1897. 

Lawson,  a.  C,  The  geology  of  the  San  Francisco  Peninsula:  Jour.  Geol- 
ogy, vol.  5,  Chicago,  1897. 

Merriam,  J.  C,  The  geological  relations  of  the  Martinez  grouj)  of  Cali- 
fornia at  the  typical  locality:  Jour.  Geology,  vol.  5,  Chicago,  1897. 

Merriam,  J.  C,  The  distribution  of  the  Neocene  sea  urchins  of  middle 
California  and  its  bearing  on  the  classification  of  the  Neocene  forma- 
tions: California  Univ.  Dept.  Geology  Bull.,  vol.  2,  No.  4,  Berkeley, 
1898. 

Turner,  H.  W.,  Notes  on  some  igneous,  metamori)hic,  and  sedimentary 
rocks  of  the  Coast  Ranges  of  California:  Jour.  Geology,  vol.  6,  Chicago, 
1898. 

Anderson,  F.  M.,  The  geology  of  Point  Reyes  Peninsula:  California 
Univ.  Dept.  Geology  Bull.,  vol.  2,  No.  5,  Berkeley,  1899. 

BlasdaIiE,  W.  C,  Contributions  to  the  mineralogy  of  California:  Cali- 
fornia Univ.  Dept.  Geology  Bull.,  vol.  2,  No.  11,  Berkeley,  1901. 

Lawson,  A.  C,  and  Pal  ache,  Charles,  The  Berkeley  Hills;  a  detail  of 
Coast  Range  geology:  California  Univ.  Dept.  Geology  Bull.,  vol.  2, 
No.  12,  Berkeley,  1901. 

Lawson,  A.  C,  A  geological  section  of  the  middle  Coast  Ranges  of  Cali- 
fornia: Science,  new  ser.,  vol.  15,  New  York,  1902. 

Anderson,  F.  M.,  Cretaceous  deposits  of  the  Pacific  coast:  California 
Acad.  Sci.  Proc,  3d  ser.,  vol.  2,  No.  1,  San  Francisco,  1902. 

ScHALLER,  W.  T.,  Minerals  from  Leona  Heights,  Alameda  County,  Cali- 
fornia: California  Univ.  Dept.  Geology  Bull.,  vol.  3,  No.  7,  Berkeley, 
1903. 

Haehl,  H.  L.,  and  Arnold,  Ralph,  The  Miocene  diabase  of  the  Santa 
Cruz  Mountains  in  San  Mateo  ('ounty,  Cal. :  Am.  Philos.  Soc.  Proc, 
vol.  43,  No.  175,  Philadelphia,  1904. 

Merriam,  J.  C,  A  note  on  the  fauna  of  the  lower  Miocene  in  California: 
California  Univ.  Dept.  Geology  Bull.,  vol.  3,  No.  16,  Berkeley,  1904. 

OsMONT,  V.  C,  Geological  section  of  the  Coast  Ranges  north  of  the  Bay 
of  San  Francisco:  California  Univ.  Dept.  Geology  Bull.,  vol.  4,  No.  3, 
Berkeley,  1904. 

OsMONT,  V.  C,  Areas  of  the  California  Neocene:  California  Univ.  Dept. 
Geology  Bull.,  vol.  4,  No.  4,  Berkeley,  1904. 

Weaver,  C.  E.,  Contribution  to  the  paleontology  of  the  Martinez  group: 
California  Univ.  Dept.  Geology  Bull.,  vol.  4,  No.  5,  Berkeley,  1905. 

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Baur,  L.  a.,  and  Burbank,  T.  C,  The  San  Francisco  earthquake  of  April 
18,  1906,  as  recorded  by  the  Coast  and  Geodetic  Survey  magnetic  observ- 
atories: Nat.  Geog.  Mag.,  vol.  19,  No.  5,  Washington,  1906. 

Taber,  Stephrx,  Local  effects  of  the  California  earthquake  of  1906:  Jour. 
Geology,  vol.  14,  No.  4,  Chicago,  1906. 

MuRGOCi,  G.,  I,  Contribution  to  the  classification  of  the  amphiboles;  II, 
On  some  glaueophane  schists,  syenites,  etc. :  California  Univ.  Dept. 
Geology  Bull.,  vol.  4,  No.  15,  Berkeley,  1906. 

Davidson,  G.,  The  San  Francisco  earthquake  of  April  18,  1906:  Am.  Philos. 
Soc.  Proc,  vol.  45,  Philadelphia,  1906. 

Davidson,  G.,  Points  of  interest  involved  in  the  San  Francisco  earthquake 
of  1906:  Am.  Philos.  Soc.  Proc,  vol.  45,  Philadelphia,  1906. 

Austin,  Mart,  The  temblor,  a  personal  narration:  Out  West,  vol.  24,  No. 
6,  Los  Angeles,  June,  1906. 

Branner,  J.  C,  Geology  and  the  earthquake:  Out  West,  vol.  24,  No.  6, 
Los  Angeles,  June,  1906. 

Gilbert,  G.  K.,  The  investigation  of  the  California  earthquake  of  1906: 
Pop.  Sci.  Monthly,  New  York,  August,  1906. 

Jordan,  D.  S.,  The  earthquake  of  April,  1906:  Pop.  Sci.  Monthly,  New 
York,  October,  1906. 

Knopf,  Adolph,  An  alteration  of  Coast  Range  serpentine:  California 
Univ.  Dept.  Geology  Bull.,  vol.  4,  No.  18,  Berkeley,  1906. 

Arnold,  Ralph,  The  Tertiary  and  Quaternary  pectens  of  California: 
U.  S.  Geol.  Survey  Prof.  Paper  47,  Washington,  1906. 

Ransome,  F.  L.,  The  probable  cause  of  the  San  Francisco  earthquake: 
Nat.  Geog.  Mag.,  vol.  17,  No.  5,  Washington,  1906. 

Hayford,  J.  F.,  and  Baldwin,  A.  L  ,  The  earth  movements  in  the  Cali- 
fornia earthquake  of  1906:  U.  S.  Coast  and  Geodetic  Survey  Rept. 
Superintendent  for  1906-7,  Appendix  3,  Washington,  1907.  Also  pub- 
lished under  the  title  Geodetic  measurements  of  earth  movements: 
Report  of  the  State  Earthquake  Investigation  Commission,  vol.  1,  pp. 
114-143,  Carnegie  Inst.  Washington  Pub.  87,  AVashington,  1908. 

Derleth,  Charles,  Jr..  The  destructive  extent  of  the  California  earth- 
quake of  1906;  its  effect  upon  structures  and  structural  materials 
within  the  earthquake  belt;  published  with  a  collection  of  reprints  of 
magazine  articles  in  a  book  entitled  The  California  earthquake  of  1906, 
edited  by  D.  S.  Jordan,  San  Francisco,  1907. 

Eakle,  A.  S.,  Notes  on  lawsonite,  columbite,  beryl,  barite,  and  calcite: 
California  Univ.  Dept.  Geology  Bull.,  vol.  5,  No.  6,  Berkeley,  1907. 

Crandall,  R.,  The  geology  of  the  San  Francisco  Peninsula:  Am.  Philos. 
Soc.  Proc,  vol.  46,  Philadelphia,  1907. 

Omori,  F.,  Preliminary  notes  on  the  cause  of  the  California  earthquake  of 
1906:  [Imperial  Earthquake  Investigation  Committee  Bull.,  vol.  1,  No.  1, 
Tokyo.  1907. 

Fairbanks,  H.  W.,  The  great  earthquake  rift  of  California:  California 
Physical  Geography  Club  Bull.,  vol.  1,  No.  2.  Berkeley,  October,  1907. 

Uhle,  Max,  The  Phueryville  shell  mound:  California  UniA^  Pub.  Am. 
Archeology  and  Ethnology,  vol.  7,  No.  1,  Berkeley,  1907. 

Smith,  J.  P.,  The  paragenesis  of  the  minerals  in  the  glaucophanebearing 
rocks  of  California:  Am.  Philos.  Soc.  Proc,  vol.  45,  Philadelphia,  1907. 


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(tILBKrt,  G.  K.,  Humphrey.  R.  L.,  Skwell,  J.  S.,  .ami  Soni.f:,  F.,  San 
Francisco  eartli(]uake  and  fire  of  April  18,  190G,  and  their  effects  on 
structures  and  structural  materials:  U.  S.  Geol.  Survey  Bull.  324, 
AVashington,  1907. 

Lawson,  a.  C,  The  California  earthquake  of  April  18,  190(5:  Report  of  the 
State  Earthquake  Investigation  Commission,  vol.  1,  Carnegie  Inst. 
Washington  Pub.  87,  Washington,  1908. 

Weaver,  C.  E.,  New  echinoids  from  the  Tertiarj  of  California:  Cali- 
fornia Univ.  Dept.  Geology  Bull.,  vol.  5,  No.  17,  Berkeley,  1908. 

Weaver,  C.  E.,  Stratigraphy  and  paleontology  of  the  San  Pablo  forma- 
tion in  middle  California:  California  Univ.  Dept.  Geology  Bull.,  vol. 
5,  No.  16,  Berkeley,  1909. 

Pack,  R.  W.,  Notes  on  echinoids  from  the  Tertiary  of  California:  Cali- 
fornia Univ.  Dept.  Geology  Bull.,  vol.  5,  No.  18,  Berkeley,  1909. 

Branner,  J.  C,  Newsom,  J.  F.,  and  Arnold,  Ralph,  Description  of  the 
Santa  Cruz  quadrangle:  U.  S.  Geol.  Survey  Geol.  Atlas,  Santa  Cruz 
folio  (No.  163),  Washington,  1909. 

Nelson,  N.  C,  Shell  mounds  of  San  Francisco  Bay  region:  California 
Univ.  Pub.  Am.  Archeology  and  Ethnology,  vol.  7,  No.  4,  Berkeley, 
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Nelson,  N.  C,  The  Ellis  Landing  shell  mound:  California  Univ.  Pub. 
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Rothplktz,  a.,  Ueber  die  Ursachen  des  Kalifornischen  Erdbebens  von 
1906:  K.  bayer.  Akad.  Wiss.,  Math.-Phys.  Klasse  Sitzungsber.  1910,  No. 
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Reid,  H.  F.,  The  California  earthquake  of  April  18,  1906:  Report  of  the 
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Smith,  J.  P.,  The  geologic  record  of  California:  Jour.  Geology,  vol.  18, 
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Jones,  W.  F.,  The  geology  of  the  Sargent  oil  field:  California  Univ.  Dei)t. 
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DiCKERSON,  R.  E.,  The  stratigraphic  and  faunal  relations  of  the  Martinez 
formation  to  the  Chico  and  Tejon  north  of  Mount  Diablo:  California 
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Reid,  H.  F.,  The  elastic-rebound  theory  of  earthquakes:  California  Univ. 
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Mkrriam,  J.  C,  Vertebi'ate  fauna  of  the  Oriudan  and  Siestan  beds  in 
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October,  1912. 


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Elmoro 

Bristol 

La  Plata 

Monterey 

Menominee  special 

Mother  Lode  district 

Uvalde 

Tintic  special -. 

Colfax 

Danville 

Walsenburg 

Huntington 


Cal 

Wyo 

Cal 

W.  Va.-Va._. 

Tenn 

W.  Va 

Ala 

Colo 

Cal 

Mont 

Gal 

Tenn 

Cal 

Tex 

Cal 

Va.-W.  Va-_ 

Idaho 

Ky 

Ky. ,__...-.,. 

Colo .-.., 

Oreg 

Mass. -Conn  _ 

Cal 

Wyo 

Tenn 

Wash 

Mont . 

Mont -. 

Colo 

Colo 

Va.-Tenn  .... 

Colo ... 

Va.-W.  Va._ 

Mich 

Cal 

Tex 

Utah . 

Cal 

Ill.-Ind 

Colo 

W.  Va.-Ohio . 


•  Order  by  number.    +  Payment  must  be  made.by.  mon«!y  order,  orin.casli.    t  Foliosout  of  stock. 


PUBLISHED   GEGT^OGIC   POLIOS— Continued. 


Name  of  folio. 


State. 


AV.ishington  .   _ 

Spanish  Peaks 

Charleston 

Coos  Bay  . 
Coalgate.   _ 
Maynardville 

Austin 

Raleigh 
Rome  _  - 

Atoka  .     

Norfolk 

Chicago :____ 

Masontown-UniontoAvn  , 

New  York  City 

Ditney 

Oelriehs 

Ellensburg 

Camp  Clarke 

Scotts  Bluff 

Port  Orford 

Cranberry 

Hartville 

Gaines 

Elkland-Tioga 

Brownsville-Connellsville 

Columbia 

Olivet 

Parker 

Tishomingo 

Mitchell 

Alexandria 

San  Luis  ^ 

Indiana 

Nampa 

Silver  City 

Patoka - 

Mount  Stuart 

Newcastle 

Edgemont 

Cottonwood  Falls 
Latrobe 


I)   C.-Va.-Md. 

Colo 

W.  Va 

Oreg 

Okla 

Tenn 

Tex 

W.  Va 

Ga.-Ala._ 

Okla 

Va.-N.  C- 
111. -Ind  _. 

Pa 

N.  Y.-N.  J-  - 

Ind 

S.  Dak.-Nebr 

Wash 

Nebr 

Nebr 

Oreg 

N.  C.-Tenn  ._ 

Wyo 

Pa.-N.  Y 

Pa 

Pa 

Tenn 

S.  Dak 

S.  Dak 

Okla 

S.  Dak 

S.  "Dak 

Cal 

Pa 

Idaho-Oreg  .  - 

Idaho 

Ind.-Ill 

Wash 

Wyo.-S.  Dak 
S.  Dak. -Nebr 

Kane 

Pa.......---. 


♦Order Ijy  number.    tPayment  must be'ifta'de byWoney  oVder  tfr  fn'oaBh.    fFolios  oiitof  stbck. 


PUBLISHED   GEOLOGIC   FOLIOS— Continued. 


Name  of  folio. 


State. 


Globe 

Bisbee  (reprint) 

Huron 

De  Smet 

Kittanning 

Asheville 

Casselton-Fargo 

Greeneville 

Faj'-etteville 

Silverton 

Waynesburg 

Tahlequah 

Elders  Ridge 

Mount  Mitchell 

Rural  Valley 

Bradshaw  Mountains 

Sundance 

Aladdin 

Clifton 

Rico 

Needle  Mountains 

Muscogee 

Ebensburg  

Beaver  

Nepesta 

St.  Marys 

Dover 

Redding 

Snoqualmie 

Milwaukee  special 

Bald  Mountain-Dayton 

Cloud  Peak-Fort  McKinney. 

Nantahala 

Amity 

Lancaster-Mineral  Point 

Rogersville . 

Pisgah 

Joplin  district  (reprint) 

Penobscot  Bay 

Devils  Tower 

Roan  Mountain 


Ariz 

Ariz 

S.  Dak 

S.  Dak .-_. 

Pa '. 

N.  C.-Tenn 

N.  Dak.-Minn 

Tenn.-N.  C 

Ark.-Mo 

Colo 

Pa 

Okla.-Ark 

Pa 

N.  C.-Tenn 

Pa 

Ariz 

Wyo.-S.  Dak 

Wyo.-S.  Dak.-Mont... 

Ariz 

Colo 

Colo 

Okla 

Pa 

Pa 

Colo 

Md.-Va 

Del.-Md.-N.  J 

Cal 

Wash 

Wis 

Wyo 

Wyo 

N.  C.-Tenn 

Pa 

Wis.-Iowa-IU 

Pa 

K.C.-S.  C 

Mo.-Kans 

Maine ^_., 

Wyo 

Tenn.-N   C 


Order  by  number.     +  Payment  must  be  made  by  money  order  or  in  cash.    {  Folios  out  of  stock 


PUBLISHED  GEOLOGIC  FOLIOS— Continued. 


Name  of  t'olio. 


State. 


Patuxent 

Ouray 

Winslow 

Ann  Arbor 

Elk  Point 

Passaic 

Rockland 

Independence  

Accident-Grantsville 

Franklin  Furnace 

Philadelphia ^- 

SantaCruz.-  

Belle  Fourche 

Aberdeen-Redfleld 

El  Paso 

Trenton 

Jamestown-Tower 

Watkins  Glen-Catatonk 

Mercersburg-Chambersburg 

Engineer  Mountain 

Warren 

Laramie- Sherman 

Johnstown 

Birmingham . 

Sewickley 

Burgettstown-Carnegie 

Foxburg-Clarion 

Pawpaw-Hancock 

Claysville 

Bismarck 

Choptank 

Llano-Burnet 

Kenova 

Murphy  sboro-Herrln 

Apishapa 

Ellijay 

Tallula- Springfield 


Md.-D.  C 

Colo 

Ark.-Okla 

Mich 

S.  Dak. -Nebr. -Iowa  .. 

N.  J.-N.  Y 

Maine 

Kans 

Md.-Pa.-W.  Va 

N.J 

Pa.-N.  J.-Del 

Cal 

S.  Dak 

S.  Dak 

Tex 

N.  J.-Pa 

N.  Dak 

N.  Y 

Pa 

Colo 

Pa.-N.  Y 

Wyo 

Pa 

Ala 

Pa 

Pa 

Pa 

Md.-W.  Va.-Pa 

Pa 

N.  Dak 

Md 

Tex 

Ky.-AV.  Va.-Ohio.--. 

Ill 

Colo 

Ga.-N.  C.-Tenn 

III 


♦Order  by  number. 

t  Payment  must  be  made  by  money  order  or  in  cash. 

t  Folios  out  of  stock. 

§  Folios  164  to  188  are  published  in  both  a  library  edition  (18  by  22  inches)  and  a  field  edition 
(6  by  9  inches).  The  field  edition  of  folios  164  to  184  inclusive  costs  6  cents ;  that  of  folios  185  to 
188  costs  50  cents.    The  field  edition  of  folio  169  is  out  of  stock. 


PUBLISHED   GEOLOGIC   FOLIOS— Continued. 


No* 

Name  of  folio. 

State. 

Price. t 

189 

Barnesboro-Patton 

Pa 

Cents. 
25 

§190 

Niagara 

N.  Y 

50 

§191 

Raritan 

N.  J 

25 

192 

Eastport 

Maine 

25 

§193 

San  Francisco i 

Cal 

75 

*  Order  by  number, 

t  Payment  must  be  made  by  money  order  or  in  cash. 

§  Folios  190,  191,  and  193  are  published  in  both  a  library  edition  (18  by  313  inches)  and  a  field 
edition  (6  by  9  inches).  The  price  of  the  field  edition  of  folios  190  and  191  is  50  cents,  and  that  of 
folio  193  is  75  cents. 


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Plate  VI  ELLIPSOIDAL  BASALT  INTRUSIVE  INTO  THIN-BEDDED 
RADIOLARIAN  CHERT  OF  FRANCISCAN  GROUP,  HUNTER 
POINT,  SAN  FRANCISCO. 

The  layers  of  chert  are  much  contorted  by  the  intrusion. 


PLATE  Vll.    THIN-BEDDED  CHERT  AND  SHALE  OF  THE  CLAREN^^^^^^^ 

MONTEREY  GROUP,  CLAREMONT  CANYON,  BERKELEY  HILLb. 

The  layers  of  shale  are  much  thinner  than  the  layers  of  chert,  which  are  white  in  the  picture. 


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Plate  X.    TRACE  OF  SAN   ANDREAS    FAULT,  MADE    BY   THE    MOVEMENT 
WHICH    CAUSED  THE   EARTHQUAKE  OF   1906. 

On  the  left  is  the  rift  valley,  which  extends  from  Bolinas  Lagoon  to  Tomales  Bay. 


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